Their condition does not mean that God's word has failed. Not everyone in the family line of Israel really belongs to Israel.
Not everyone in Abraham's family line is really his child. Not at all! Scripture says, "Your family line will continue through Isaac."(Genesis 21:12)
In other words, God's children are not just Abraham's natural children. Instead, they are the children God promised to him. They are the ones considered to be Abraham's children.
God promised, "I will return at the appointed time. Sarah will have a son."(Genesis 18:10,14)
And that's not all. Rebekah's children had the same father. He was our father Isaac.
Here is what happened. Rebekah's twins had not even been born. They hadn't done anything good or bad yet. So they show that God's purpose is based firmly on his free choice.
It was not because of anything they did but because of God's choice. So Rebekah was told, "The older son will serve the younger one."(Genesis 25:23)
It is written, "I chose Jacob instead of Esau."(Malachi 1:2,3)
What should we say then? Is God unfair? Not at all!
He said to Moses, "I will have mercy on whom I have mercy. I will show love to those I love." (Exodus 33:19)
So it doesn't depend on what we want or do. It depends on God's mercy.