Romans 9:12–13:

not by works but by him who calls—she was told, “The older will serve the younger.” Just as it is written: “Jacob I loved, but Esau I hated.” (NIV)

it was said to her, “THE OLDER WILL SERVE THE YOUNGER.” Just as it is written, “JACOB I LOVED, BUT ESAU I HATED.” (NASB)

It was said unto her, The elder shall serve the younger. As it is written, Jacob have I loved, but Esau have I hated. (KJV)

In Rom 9:7–11 Paul has been arguing 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.” In Rom 9:11, Paul specifically states the reason for his proof is “in order that God’s purpose in election might stand.”

Now, Paul simply provides more proof for his argument via Scripture: “According to natural birth order, it seems plausible that the younger would serve the older; God said to Rebekah that ‘the older will serve the younger (Gen. 25:23)’; therefore, God’s promises are not bound by natural birth implications.”

Also, “It seems natural that there would be no reason to love one brother, but hate another; God said ‘Jacob I loved, but Esau I hated’ (Mal. 1:2,3); therefore, God’s favor is not bound by natural relationships.”

Both of these are to show that it is not only plausible, but also necessary that it is according to God’s election and promise—i.e., not familial relationship or inheritance—that God’s children are determined.