The OT carries a lineage marker that corresponds to the hope that Adam and Eve had as a redeemer to restore them back to right standing with God. From Adam to Noah and onwards Abraham down the line there is a seed through which the Messiah would come.
The את is used in other ways too but when attached to a name it appears to show lineage.
“I will put enmity between you and the woman, and between your offspring and her offspring; he shall bruise your head, and you shall bruise his heel."”
He being the Messiah.
Initially Eve thought it was Cain and when reading in the Hebrew Cain carried the et marker attached to his name but after murdering Abel it moved to Seth.
“Now Adam knew את Eve his wife, and she conceived and bore את Cain, saying, "I have gotten a man with the help of the Lord."”
The marker goes through the woman because it was her seed and Cain picks it up.
V2 shows Abel picking it up too as if to denote it could go through both lines. Or in anticipation of what would follow with his murder because Cain no longer has the את next to his name.
“And Adam knew his wife again, and she bore a son and called his את name Seth, for she said, "God has appointed for me another offspring instead of Abel, for Cain killed him."”
Cain’s lineage no longer bears the את
“To Seth also a son was born, and he called his את name Enosh. At that time people began to call upon the name of the Lord.”
You get the idea. Skipping over to Abraham
Abram has a first born Ishmael through Hagar
“Abram was eighty-six years old when Hagar bore את Ishmael to Abram.”
The next times Ishmael’s name appears he no longer has the covenant marker by his name. It is dropped off entirely.
(The marker does appear again in conjunction with the boy and the mother as a covenant marker that God will honor the voice of the mother for the child because of the binding covenant He has made to preserve Ishmael but not a lineage indicator.)
Covenant or seed
“And I will establish my את covenant between me and you and your offspring after you throughout their generations for an everlasting covenant, to be God to you and to your offspring after you.”
And now Isaac appears in the picture and the marker reappears in the same verse next to both Isaac’s name and God’s covenant promise.
“God said, "No, but Sarah your wife shall bear you a son, and you shall call his את name Isaac. I will establish my את covenant with him as an everlasting covenant for his offspring after him.”
Your verse in question.
“But God said to Abraham, "Be not displeased because of the boy and because of your slave woman. Whatever Sarah says to you, do as she tells you, for through Isaac shall your offspring be named.”
We could go to the NT and Paul explains it by saying it’s singular and not plural referring to the Messiah.
“Now the promises were made to Abraham and to his offspring. It does not say, "And to offsprings," referring to many, but referring to one, "And to your offspring," who is Christ.”
Yet even Paul has to prove how he got that and it goes back to Eve and the curse on the serpentine creature. There would arise a Messiah.
Isaac has two children, twins.
Interesting that the first time they are mentioned with the marker they both carry it
“Isaac loved את Esau because he ate of his game, but Rebekah loved את Jacob.”
This would be explained with extraBiblical texts and the promises made to Abraham about the two boys, except we can see in this verse that Isaac had every intention to give Esau the birthright blessing to carry the seed forth which Rebecca was holding to the promise prophesied
“And the Lord said to her, "Two nations are in your womb, and two peoples from within you shall be divided; the one shall be stronger than the other, the older shall serve the younger."”
The marker next to birthright
“Jacob said, "Sell me your את birthright now."”
Birthright is in conjunction with Esau.
V33 birthright appears in conjunction with Jacob now and no longer with Esau.
The blessing of Isaac to the firstborn
“Rebekah said to her son Jacob, "I heard your את father speak to your brother Esau,”
Isaac carries the lineage marker.
Then Jacob and his mother complot against Esau as soon as Isaac finishes the blessing Jacob appears with the marker next to his name. And Esau no longer has it next to his.
“As soon as Isaac had finished blessing את Jacob, when Jacob had scarcely gone out from the presence of Isaac his father, Esau his brother came in from his hunting.”
The lineage comes through Jacob for the Messiah. Obviously this continues on but the question was about Isaac’s name. There is a curious event that happens in Ruth, as soon as Boaz takes her she picks up the את and she gets named in the genealogies of the NT. It’s important to note that the את doesn’t follow the rule of grammar for direct object pointer in every instance when one is required and also appears where a direct object point should not appear.