Peter is speaking of the fruit of her (the woman's) womb by alluding to Psalm 127:3.
The word Peter uses for "heirs together" is συγκληρονόμοι which is a "compound word" in that it consists of a prepositional prefix plus a noun.
In Psalm 127:3 the word is κληρονομία showing that children are a gift from God and his "heritage" (that which he gives his heirs).
In the ancient world, before contraception, sexual relations would generally speaking lead to babies. However, the Psalm is intent on driving home that it is only YHVH that can produce life and that one can have sex all day and night but unless the LORD gives life it was all for nothing (well, maybe not nothing, but it isn't going to fill your quiver):
[Psa 127:1-3 KJV] (1) [A Song of degrees for Solomon.] Except the LORD build the house, they labour in vain that build it: except the LORD keep the city, the watchman waketh but in vain. (2) It is vain for you to rise up early, to sit up late, to eat the bread of sorrows: for so he giveth his beloved sleep. (3) Lo, children are an heritage of the LORD: and the fruit of the womb is his reward.
So the "grace of life" is the miracle of birth that only God can give.
The reason he is reminding the husbands to be considerate of a woman's weakened condition is because she has been bearing their fruit, the heritage of the LORD:
4 The bows of the mighty have become weak, and those who were
weak have been clothed with might. 5 Full of bread, they suffer
loss, and being weak, they neglect the land. For the barren
woman bore seven, and the woman who is many in children has become
Brannan, R., Penner, K. M., Loken, I., Aubrey, M., & Hoogendyk, I. (Eds.). (2012). The Lexham English Septuagint (1 Kgdms 2:4–5). Bellingham, WA: Lexham Press.
She was weakened by childbirth and so God pays her a reward:
[Psa 127:3 KJV] (3) Lo, children are an heritage of the LORD: and the fruit of the womb is his reward.
The word translated "reward" is μισθὸς which basically refers to "a reward for service". Since both the man and the woman inherit from the LORD both an inheritance and the reward for her childbirth, the man needs to pitch in too. Not so much with the birthing (unless you count the cheer-leading) but by living considerately and appreciatively for her giving her very life's energy to the young "arrowlet".