Romans 9:10–11:

Not only that, but Rebekah’s children had one and the same father, our father Isaac. Yet, before the twins were born or had done anything good or bad—in order that God’s purpose in election might stand: (NIV)

And not only this, but there was Rebekah also, when she had conceived twins by one man, our father Isaac; for though the twins were not yet born and had not done anything good or bad, so that God’s purpose according to His choice would stand, not because of works but because of Him who calls, (NASB)

And not only this; but when Rebecca also had conceived by one, even by our father Isaac; (For the children being not yet born, neither having done any good or evil, that the purpose of God according to election might stand, not of works, but of him that calleth;) (KJV)

In Rom 9:7–9, Paul uses the example of the children of Abraham and Sarah to show that “It is not the natural children who are God’s children, but it is the children of the promise who are regarded as Abraham’s offspring, i.e., God’s children.”

Now, Paul moves to the next generation to offer a potentially more convincing—or, at the very least, clear—example. Whereas Isaac and Ishmael are born of different parents (Abraham having Ishmael from Hagar, Isaac from Sarah), Jacob and Esau had the same parents: Rebekah and Isaac (Rom 9:10). This removes the variable of differing parents with respect to inheritance and the promise.

Paul also removes the variables of birth order and works, i.e., “before the twins were born or had done anything good or bad” (Rom 9:11).

This further proof is offered “in order that God’s purpose in election might stand,” i.e., that it can not be mistaken that “It is not the natural children who are God’s children, but it is the children of the promise who are regarded as Abraham’s offspring, i.e., God’s children.”