Albert Barnes Commentary Romans 9:10

"And not only so; but Rebecca also having conceived by one, [even] by our father Isaac--" — Romans 9:10 (ASV)

And not only this. Not only is the principle of making a distinction among the natural descendants of Abraham thus settled by the promise, but it is still further seen and illustrated in the birth of Isaac’s two sons.

He had shown that the principle of making this distinction among Abraham’s descendants was recognized in the original promise, thus proving that not all of Abraham’s descendants were necessarily to be saved; and he now proceeds to show that the principle was also recognized among Isaac's descendants, within his own family.

And he shows that the selection was not made according to any natural principles. He not only made a distinction between Jacob and Esau, but he did it according to his good pleasure, choosing the younger to be the object of his favour and rejecting the elder, who, according to the customs of that time, was supposed to be entitled to special honour and rights.

And, to prove that this was done according to his own pleasure, he shows that the distinction was made before they were born, before they had formed any character, and consequently, in such a way that it could not be claimed it was because of any works they had performed.

But when Rebecca. She was the wife of Isaac. See Genesis 25:21, 23.