Preparing the Way:
The OP asked two questions under the auspice that incest between brother and sister is a sin (Leviticus 20:17); Q1: Who did Cain marry? and Q2: How did God intend for man to "Be fruitful and multiply" without sinning? In answering these questions individually or collectively, there is no great concern if we answer them under the narratives formed by Dispensationalism or Covenant Theology. Each framework insists that God dispenses with sins until the appropriate hour when he prohibits such sins.
What is problematic about these frameworks is that they presume (Deuteronomy 18:20-22). And what they presume is that God acts without precedence or warning. Such as with Cain's wife, the argument follows that until the population of the earth was of such a particular number, that it was then God found it safe to prohibit incest. What was that number? When was that number reached? How is such a number of God when there are endogamic tribes today which observe prohibitions against incest and thrive ("Cousin Marriage", The Northern Bantu, Roscoe, pg. 38). The inference being, there was no need for hundreds of thousands of people, only a hundred or so by at least four generations.
And yet, even when we consider this minuscule number of a population, there looms a larger problem borne in the OP's question: What kind of God impresses on man to resist the temptation of sin, and yet indulges in sin when he pleases? How can such a god exhort man to cleanse and prepare for himself a bride without spot or wrinkle (Ephesians 5:27), when he's allowed the man's fathers to dabble in the sins he now forbids? It is a god who cannot merit the honor as ALPHA and OMEGA.
As for Q2, answering this requires us to return to the events that take place on the Sixth Day of creation (Genesis 1:24-27). As it is here we're best to reckon who, and not so much how, God envisioned propagating the earth.
A Taste of Eden:
Answering "Who did Cain marry", is a difficult and perplexing pursuit. The reason, is because we traditionally enter the Scriptures under the premise that, when the LORD God created man from the dust of the earth, this was the seminal moment when God created the first of all mankind (Genesis 2:7). This presumption is why we conflate the 1st chapter of Genesis with the 2nd chapter (Genesis 2:4-25).
It is an ethic that distorts Scripture in its entirety. Reckoning the Bible's Creation as a metaphysical creation, allows us to reckon all things. It says that, beyond Genesis 1:1, God did not concern Himself with a physical creation. In Genesis 1:1 that God established the pillars, "the Heaven and the earth", through which all things enter and return from the Universe. It is these pillars that account for all that enters, as they weigh that account against all things as they return. The conclusion being, it is in Genesis 1:1, God creates all that concerns the physical world.
With man, both male and female, being in existence. As a challenge to our fathers' tradition, what we find taking place in the 2nd chapter of Genesis is where the LORD God establishes Eden to serve as a Temple in which to train up High Priests. Eden is not the entirety of the Earth. Eden is a sliver, carved out of the Earth for the purpose in training up a Divine Priesthood.
Where the LORD God:
7 ...formed man of the dust of the ground, and breathed into his nostrils the breath of life; and man became a living soul. (Genesis 2:7 KJV)
This alludes to the effort made by the LORD God to weigh the preparedness of those men he found, in the tribes and nations throughout the earth, as for their spiritual fitness to endure the rigors of Eden. To form a "man of the dust of the ground" is not in the sense of forming the mold of a man. This alludes to bringing a man into the awareness of himself in respect to the ancestral legacy that substantiates his being and presence. The reason this perspective in necessary in determining a man's fitness, is because as the LORD God brings him into this awareness. It allows the initiate perspective so that he's better to see how it is the male; the man; the husband and father who bears the soul that lives. Lives, in the sense, that it is the man who's able to justify his living as a beast or as a man. The object of the metaphysical creation is for man to reckon what behaviors differentiate him from the beasts of the field.
The action in making the initiate aware of this, is when the LORD God "blew" in the man's nostrils. Why the nostrils? This is symbolism in how the LORD God quickened the soul of the initiate. Our nostrils serve as the pathway to what is the olfactory system, the sensory system is where we arrive at our likes and dislikes, habits, and beliefs. What the LORD God sought to make the initiate aware of, is how revelation falls to the man, and thus it is on the man does all hang. Because it is the only soul that lives, it is the man's soul that is able to experience and realize death. The death of the soul being the proverbial death, as this death also condemns the body.
Our assurance comes in what takes place in the wake of Adam's fall. Adam, so named, because he is the first of all the initiates to fail in the Garden in such a calamitous manner. Whether the second or the first of thousands, when a man fails to "dress" the Garden in a manner that was in keeping with Eden (Genesis 2:15). It is then
23 ...the Lord God sent him forth from the garden of Eden, to till the ground from whence he was taken. (Genesis 3:23).
Realizing The HOLY GHOST:
How this pertains to Cain and his wife, is that it opens the way for us to see that Cain and Abel were not biological, but spiritual sons of Adam. Cain and Abel were priests. They were priests, in the sense, that Adam divided the office and obligations of the High Priest. It was an effort on Adam's part to repent for what transpired in Eden. How Adam sought to repent was that he fashioned Cain in the image of the Serpent, which was unique (Genesis 4:1). While Abel was merely to officiate in the theological and ceremonial aspects of the religion.
The difference in the labors of Cain and Abel, signify the essences laden within any religious tradition. Paul offers us a context of this where he speaks to things as being "in part", and then, over time, we realize that which is perfect that comes and resolves those things once in part:
9 For we know in part, and we prophesy in part. 10 But when that which is perfect is come, then that which is in part shall be done away. (1 Corinthians 13:9-10)
Where the 9th verse alludes to an excited utterance, where we declare what is and is not of God. The 10th verse alludes to depth of history man endures to realize the verity of what their religious or ancestral fathers declared. Where, in the wake of that which is perfect comes and resolves all that was once in part. It then falls upon the religion to reckon itself with the revelation, else the religion falls into chaos.
In Adam's dividing up his priesthood, he assigns the spiritual and religious aspects to Abel; the excited utterance and the obligation to reconcile that which is perfect. The temporal aspects, which bear witness to when that which is perfect comes, falls to Cain.
It is in this manner are we better in reckoning the divisions of labor between Cain and Abel, whereby:
2 ...Abel was a keeper of sheep, but Cain was a tiller of the ground. (Genesis 4:2)
The same serves for why Scripture alters the birth order of Cain and Abel. It serves to note how Cain was Adam's effort at repentance. Adam sought to recreate the spiritual environs of Eden. And to do that, it required recreating the presence of the Serpent. Never to appreciate how such a recreation served as a provocation, given the Serpent bears witness to man as the Spirit or Angel of Truth (AOT).
As for the Serpent as the AOT. Man is the Serpent's sustenance. Man is the sustenance by which the Serpent lives and has life. In his turning to dust, this dust signifies the capturing of every aspect of a man's life, both word and deed, what's hidden as well as what's open. In the Serpent's consuming the dust, He thus becomes the embodiment of the truth that follows man.
Prevailing as the AOT was the Serpent's purpose from the beginning. In the LORD God's effort to capture the essence of the AOT, the LORD God reckoned the Spirit of the AOT in the form of a serpent.
Why a "serpent"? In the sense of Scripture a serpent takes on the presence of a teaching or belief. The same is the preoccupation with Moses where the LORD finds him overly distressed about whether or not the leaders of Israel will hear him (Exodus 4:1-4). It is for this reason the LORD trains up Moses to take up "serpents", in that the power of a belief or teaching lies in its origins, just as the strength and power of a snake lies in its tail.
The same serves as the allusion to why the LORD commands Moses to take the serpent by its tail. In taking up its strength the serpent then becomes a "rod" (Exodus 4:4). That is, the particular belief or teaching, not attends in the holder's hand as a tool for instruction, chastening, or disciplining.
So, as the embodiment of the AOT, why Adam seeks to fashion Cain in the image of the Serpent, is because Adam came to realize how the Serpent was an invaluable asset. Through the woman, when the Serpent impressed on Adam how eating the fruit would not result in a natural death. But that eating the fruit would be the expedient way to unveil how good and evil stood in the sight and presence of GOD. It is for this cause Adam sought and ate what the LORD God forbade, as the Law of Eden (Genesis 2:16-17) had become for Adam an exasperating temptation. The same is why Adam contrived for himself his own law.
In contravention to the LORD God's commandment, Adam trained up the woman in how he envisioned best going to and fro throughout the whole of Eden. How Adam sought to do so was to evade the forbidden fruit at all costs, which is why he impressed on the woman the lie that God hath said:
3 But of the fruit of the tree which is in the midst of the garden...Ye shall not eat of it, neither shall ye touch it, lest ye die. (Genesis 3:3).
So, when the Serpent revealed what results in eating what the LORD God forbade, to establish himself in trespass, all Adam had to do was touch the fruit. Eating it only then becomes the appropriate next step.
Yet, over time, now cast out of Eden. Adam came to realize the truth borne by the Serpent. And in being aware of how the LORD God revealed to him how he and every generation after him was to look upon the Serpent and His presence (Genesis 3:14-15). Adam believed he could preserve the same in Cain, as a tiller of the ground, since:
14 ...the Lord God said unto the serpent, Because thou hast done this, thou art cursed above all cattle, and above every beast of the field; upon thy belly shalt thou go, and dust shalt thou eat all the days of thy life. (Genesis 3:14)
It is for this cause Adam fashioned Cain as a "tiller of the ground" (Genesis 4:2). It was an act in haste because Adam failed to recognize the latter element of what the LORD God revealed in the Serpent's relation to man and his religion, in that it is the LORD who:
15 ...will put enmity between thee and the woman, and between thy seed and her seed; it shall bruise thy head, and thou shalt bruise his heel. (Genesis 3:15).
The harrowing implications being, where Cain stands in the image of the Serpent, it is Abel who stands as the image of the woman, as this woman is the vessel of a religion. Later we discover how enmity emerges between the "Serpent" and the "woman" in the figures of Cain and Abel, when they seek to prove whose majesty stood greater in the sight of the LORD; that of the ground (Cain-the Serpent) or that which tended to sheep (Abel-woman).
These relations allow us to see why the LORD had respect unto Abel and Abel's offering, apart from Cain and his offering. Through the effects of religion, we have the pathways of forgiveness and repentance. While, that which we find laden in the AOT, the Serpent, and Cain only serve as the cause as for why or what is the way unto repentance. Since, borne in the Serpent is the Spirit of Truth, or better, the Holy Ghost.
The Nature and Origins of Cain's Wife:
Now we can consider the nature and origins of Cain's wife. Having come to understand what finds us at the brink of Cain murdering Abel. It is a legacy that has its origins in Adam's figuring Cain in the image of the Serpent. Where to recreate the spiritual environs of Eden, Adam fashions Cain as the Serpent in that Cain's a tiller of the ground. An action done in haste because there is a caveat that follows the Serpent, in that, it is the LORD who establishes enmity between the Serpent and the woman. This woman serves as the presence of the man, in that she is the embodiment of the man's religion and religious beliefs.
As for the presence of this caveat in the 4th chapter of Genesis. It comes to the fore when we find Cain and Abel pitted against each other as for who has the better stance in the eyes of the LORD.
Our moment of enmity is when the LORD has respect unto both Abel and Abel's offering, but not to Cain and Cain's offering. As for this caveat's fruition, it is a moment of enmity that leads to Cain slaying Abel. What serves as the object of Cain's animosity, is the belief that he's defending the LORD. It is a defense, in the mind of Cain, in that, Cain becomes aware of Abel's efforts to employ the LORD as a tool of division between himself and Adam. Signified in their offering since, when Cain makes his offering, Cain does so unto the LORD. But when Abel makes his offering, he does solely for the purpose to provoke Cain.
That is, though Abel's offering was a blood offering, the spirit in which Abel made his offering was to the glorification of himself. Abel did not present his gift unto the LORD, in that:
3 ...in process of time it came to pass, that Cain brought of the fruit of the ground an offering unto the Lord. 4 And Abel, he also brought of the firstlings of his flock and of the fat thereof. (Genesis 4:3-4)
So, it is Abel who serves as the provocation which leads to why we find these priestly brothers seeking the LORD's approval as for who is greater in His eyes.
The same is the reason why the LORD "had respect" unto Abel and his offering, and "had not respect" unto Cain and Cain's offering. It is not that the LORD accepted or rejected either. To accept and reject express what the LORD will and will not take unto Himself. If this were the reality, then it'd spell ruin for Cain, as he'd find himself encountering the LORD's wrathful judgement in the wake of his offering. By the LORD having "respect" in either sense, He establishes for Himself a distance between each: a "sacred space" the LORD affords that we repent.
Aware of the result, Cain having come to the realization why Abel put forth the challenge. Where, as for Abel, with Adam having set him aside. Knowing the LORD to be the greater arbiter. Abel put forth the challenge as a response to how Eve celebrated the "birth" of Cain, but considered Abel's birth mundane and trite. To affirm his greatness over his spiritual brother, Abel sought to weigh the matter in the eyes of the LORD.
So, when Cain takes up the matter to slay Abel, Cain justifies his action under the guise of defending LORD. A cause, for which, Cain sought to wage what today is a holy war, though this war consumed only Abel.
It is for this cause, Cain having chosen how to partake in the "sacred space", that we find Cain cast out of the land which bore Eve and the Adam's priesthood. Being cast out, Cain then attends in the earth to establish for himself a wife.
This wife is not Cain's sister, though she is a daughter of Eve's. How we arrive at this juncture centers on the effects of Genesis 3:16, where the LORD said unto the woman:
16 Unto the woman he said, I will greatly multiply thy sorrow and thy conception; in sorrow thou shalt bring forth children; and thy desire shall be to thy husband, and he shall rule over thee. (Genesis 3:16)
Now we enter difficulty when we interpret "sorrow" as physical pain, instead of reckoning all through the lens of "sorrow". What this sense of "sorrow" alludes to is the experience a Mother religion goes through with the birth of a sectarian offspring. Such is in the likeness of the Catholic and Protestant religions. Where the Catholic is our Mother, the Protestant is her daughter. In his work, The Jewish Religion: A Companion, Louis Jacobs makes such a comparison to establish how Judaism, the Mother, envisions Christianity and Islam as daughter religions (pg. 264).
What then serves as the origins of Cain's wife. Having reckoned Cain as a priestly figure, Cain's wife is a wife born in the manner we find in the legacies of Martin Luther, John Calvin, and Henry VIII. Again, how we reckon this birth is in what the LORD God says to the woman, as a result of Adam's fall. The LORD God impresses on the woman that it is He who:
16 ...will greatly multiply thy sorrow and thy conception; in sorrow thou shalt bring forth children; and thy desire shall be to thy husband, and he shall rule over thee. (Genesis 3:16)
Genesis 3:16 and the Births of Sons and Daughters
So that we are able to understand Cain's wife, we must first reckon Genesis 3:16 with those in the era of the Reformation. As it concerns Luther, Calvin, and Henry VIII, each took unto themselves a wife. We know their wives by the names Calvinism and Presbyterianism, as it concerns John Calvin; Lutheranism, as it concerns Martin Luther; and Anglicanism, as it concerns Henry VIII. What we must be mindful of is how they attend as their "wives" is in the same since as the "woman" attended as Adam's "wife". It is a result of they having named her.
That is, there is a gulf between the "Mother" and "wife" wherein we find the daughter. It is in this gulf where her suitor reckons the "woman" who is to be his wife. Scripture appeals to this moment when:
22 And the rib, which the Lord God had taken from man, made he a woman, and brought her unto the man. (Genesis 2:22)
That is, here, in respect to all Adam committed to the earth, in his naming every beast and fowl the LORD God made and set before him to see what Adam might call them (Genesis 2:19-20). Where the name by which Adam called them gave each animal "being" in Adam's reality. With this result, the LORD God fashions for Adam the rudiments of a religion, the framework, of which He intimately understood to be a woman.
Being mindful that what the LORD God sought for Adam was a "help meet" (Genesis 2:19, 20) that'll aid Adam in meeting with success in Eden. Having eliminated the beast and fowl with which Adam held some intimacy of knowledge. Aware that none of these entities served as a worthy resource for revelation. The LORD God then attends to what holds the greatest of mystery and intrigue for man, which is, the natural woman. But in fashioning her as a religion, so that Adam reckons the woman's constitutional make-up, whereby he reckons what makes the woman the church.
In Genesis 2:22, when the LORD God removes the rib from the man, this "rib" being the "eye" of the soul, the property of man's soul that makes his soul "living". Having removed this rib, it is then the LORD God sews up what remains of "Adam". Whereby, in the absence of this rib, the LORD God makes what He knows is a woman.
So, when the LORD God makes what He knows is a woman. It is then the LORD God seeks to witness what Adam might call this effect, since, by whatever name Adam calls her, this will be her name. Looking upon these effects:
23 ...Adam said, This is now bone of my bones, and flesh of my flesh: she shall be called Woman, because she was taken out of Man. (Genesis 2:23).
What's important about Adam's naming of the woman "Woman", is that it is not conjecture. It is not a name Adam assumes. Looking upon her effects, Adam comes to a rationalization that substantiates the woman's presence and meaning, in that he says, "This is now bone of my bones, and flesh of my flesh" (Genesis 2:23).
Now, so that we reckon ourselves with this woman as the embodiment of a religion. Here, when it comes to the phrase "bone of my bones", what we discover is that the author's word choice is not one that alludes to the natural body. In the context of Genesis 2:23, as it concerns "bones" the author chooses (6106) instead of (1634). In this context, bone (6106-6105; Genesis 2:23) is
etsem (eh'tsem), and means "a bone" as an allusion to "strong", in the figurative sense it means "the substance". This "bone", at its root (6105; Genesis 2:23), isatsam (aw-tsam'), and means "to bind fast, i.e. close (the eyes); intransitively, to be (causatively, make) powerful or numerous". That is, this use of bone (6106-6105; Genesis 2:23) alludes to what is spiritual.
Yet, when it comes to the other choice of "bone" (1634-1633; Proverbs 17:22), this bone appeals directly to the concerns of a natural woman. In this context, bone (1634-1633; Proverbs 17:22) is gerem (gheh'-rem), and is a direct reference to "a bone (as the skeleton of the body)". And, when we consider its root (1633; Proverbs 17:22), the etymology reassures us of a natural bone, the efficacy of this bone follows the word garam (gaw-ram'), and means "to be spare or skeleton-like".
When applied to the words of Adam, who says, "This is now bone of my bones, and flesh of my flesh" (Genesis 2:23). Adam presents to us is his rationalization upon which he draws the woman near to him, as he reckons her with himself. The same serves to gird Adam in his reasons for why he calls her "Woman", the rationale being, "because she was taken out of Man" (Genesis 2:23).
Now, it is for us to recognize the "space" between when the LORD God forms the woman and Adam names her "Woman". What the LORD God does affects Him in the eyes of Adam as his "Mother", a Mother, in the same sense the Roman Catholic Church served for Calvin, Luther, and Henry VIII. This "space" between the Mother and the wife, is where we find the daughter born from the Mother.
With Henry VIII as our example. Because Henry desired a male heir to the follow him as king of England, Catherine of Aragon being unable to give him a male child. Henry sought to divorce Catherine. Yet the Catholic Church denied him his divorce. It is for this cause, though a son of the Church, Henry began to seek avenues within Scripture to argue the justification for his divorce. In this effort, it is Henry who fostered for himself a woman, which recognizes that "space" between the "Mother" and "wife".
In this interim, it is here Henry and his allies, steadily devise his new bride. Where, once he'd achieved what he believed elevated the king in supremacy, he having instilled in this woman his religious beliefs. Satisfied with her as "bone of his bones" and "flesh of his flesh". It is then Henry named her "Anglicanism", as he took her for his wife.
It is in this same sense we reckon Cain and his wife. A priestly figure, when cast out of Adam's priesthood, it is then Cain enters that "space" between "Mother" and "Wife", a space scripture reckons Cain as dwelling in the "land of Nod". Nod is not a matter of geography, but reflective of Cain's spiritual state of confusion and wandering. It bespeaks to his efforts of trying to gather himself, as a result of all that's transpired. When he finally settles upon a belief system that serves to restore him, in recognizing that belief system, in the same sense as Henry VIII, is when Cain takes unto himself a wife.
Cain's wife attends as a very revealing source of her husbands state of resolve, given Cain's learned from the failings of his father in Adam. He's learned, and repented, in that his wife does not bear him two sons. Cain's wife does not bear a son that acknowledges him. Rather:
17 ...Cain knew his wife; and she conceived, and bare Enoch: and he builded a city, and called the name of the city, after the name of his son, Enoch. (Genesis 4:17)
What is revealing about the name Enoch, elevates why Cain built a city. Enoch's (2585-2596; Genesis 4:17) name is Chanowk (khan-oke'), and means "initiated". It bears the efficacy (2596; Genesis 4:17) of being "trained up" by Cain. How we bind all of this to Genesis 4:16-17, is that, in presenting himself as high priest to a land and people who had no sense as for the Adam and Eve, the Divine priesthood and religion. As a son of Eve, Cain went into another land to establish a daughter-religion of Eve, Enoch being the first High Priest of Cain's newly found religion. The need and purpose for Cain "building a city" is to provide Enoch with the fundamental beliefs and practices of the religion.
What is so special about the births of such sons and daughters; husbands and their wives, is that no longer do we need to rely on speculation as for how a man lived for nearly a 1,000 years. Understanding the birth of a priest who stands in the image and likeness of his father (his priestly predecessor), allows us to reckon how, though there may be many priests within a priesthood. Over the course of time there shall emerge one who distinguishes himself from among the rest, in the same likeness as Peter distinguishes himself from among the other eleven disciples.
Where, as it pertained to the patriarchs, there emerged from among these numbers of priests who bore the image and likeness of their father-patriarch, that he surpasses his father in revelation and therefore serves as the predicate for the father's "death".
We must be aware, as it pertains to this age of the patriarchs where we find other sons and daughters. Here, is the basis for what we find with the Protestant traditions, but under a different auspice. Where the sons and daughters of the Biblical Patriarch represent those religions (daughters-wives) and their founders (sons-husbands) as straying from the image and likeness of their father-patriarch. With the Protestant traditions there is an effort afoot to recover the light of Jesus extinguished by our Mother in the Roman Catholic Church.
Propagating the Earth
Resolving who Cain marries in this manner, preserves God as ALPHA and OMEGA. Doing so, we reckon that incest, by whatever degree, was a sin from the beginning.
As it concerns Q2, given the commitment to interpret and understand the 4th chapter of Genesis through the lens of priests and priesthoods. This infers that the Bible's Creation is a creation concerned with metaphysics and not with creating the natural world. It is a concern with God assigning a name to all that He created.
We arrive at this approach to the Bible's Creation by two salient concerns of discussions taking place on HS. The first concerns the accuracy of Genesis 1:1's interpretation. The latter concerns "Meaning of (et)“את” and (v'et)“וְאֵ֥ת” in Genesis 1:1".
As for the first, all centers on the argument of Robert D. Holmstedt. In his "The Restrictive Syntax of Genesis i 1" (http://individual.utoronto.ca/holmstedt/Holmstedt_GenesisRelative_VT2008.pdf), he argues whether ראשׁית re'shiyth (ray-sheeth'), translates properly as "THE Beginning". I agree that ראשׁית re'shiyth (ray-sheeth') does not translate into "THE Beginning". But it is appropriate as "The beginning".
All centers on the nature of Hebrew letters. The original 22 letters of the Hebrew Alphabet are ontological letters. Each letter has an ontological reference which essentially makes its symbolism static. Unlike the English, which has 26 utterances, it has 104 symbols. As for these 104 symbols, there are two forms upper and lower; and two types Formal Cursive. The difference between the Hebrew and English is that where the Hebrew requires understanding, the English requires belief. You reckon the English as "belief" in the sense that: If you show me a letter, I'll say that it is not, and we'll both be correct. If you show me the letter "A", I'll argue that this is "a" and we'll both be correct.
So, Holmstedt falls into a snare when he attempts to translate ראשׁית re'shiyth (ray-sheeth') into English. Instead, given the specialized nature of the Hebrew letter, time is better spent trying to reckon how this word impacts our understanding of Creation. We reckon it as "The beginning", in that it alludes to a moment when God desired to see and bear witness to Himself. How this impacts creation is that this godly desire is the same desire born with every child. It is the only knowledge that is "a priori". It is a knowledge that is, and cannot be taught or mollified. It is the most primeval of all human desires.
This leads us to the latter concern for the "Meaning of (et)“את” and (v'et)“וְאֵ֥ת” in Genesis 1:1". How we settle the appearances of this word, which translates as "consent", is that we reckon how in the beginning God consented Him image reckoned as "the Heaven and the earth". With this image, God sought to weigh all that entered the earth against what returned from out of the earth. The relevance is that when God created man in His image, the pronouns "him" and "they" have the same consent. And it is in this image that God reckoned man to propagate, as it is an image within which God's Spirit ever manifests.