While Abraham had many descendants, God chose Jacob (Rom 9:11-13; Mal 1:2-5) as the line through which the covenant would be passed. The nation of Israel arises from Jacob (Gen 35:1-15), and God identifies Himself as the God of Abraham and Isaac and Jacob (Ex 3:15).

Here for the first time, God chooses between two siblings of the same Patriarch (Rom 9:10-13).

In loving commitment to His covenant promises to Abraham, God chooses who will fulfill His promises and carryout His covenant plan. As Paul, the former teacher of the Law, defines who the "children of the promise" are to the church of Rome, he makes clear that God's sovereign choice of instruments of the covenant is based on love for the one who has faith in Him:

But it is not as though the word of God has failed. For they are not all Israel who are descended from Israel; nor are they all children because they are Abraham's descendants, but: "through Isaac your descendants shall be named." That is, it is not the children of the flesh who are children of God, but the children of the promise are regarded as descendants. For this is the word of promise: "At this time I will come, and Sarah will have a son." And not only that, but there was also Rebekah, 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, it was said to her, "The older will serve the younger." Just as it is written: "Jacob I have loved, but Esau I have hated." (Rom 9:6-13)

Isaac, Jacob, Moses, David, and Jesus are examples of principal faithful individuals, whom God loved, who were all chosen to mediate God's covenant promises.

For deeper study:

What does "election" mean in the Old Testament?