Paul is building a powerful argument about God's sovereign choice. After mentioning Ishmael and Isaac, he moves to an even more compelling example. Why is this next case so crucial for his point?
The apostle Paul uses the story of Jacob and Esau to demonstrate that God's selection isn't based on natural lineage or outward circumstances. While Ishmael and Isaac shared only one parent (Abraham), and Ishmael was born of a servant, Jacob and Esau were twins, born of the same father (Isaac) and the same mother (Rebekah), and to a lawful wife.
This pairing is vital because it removes potential objections. If God's choice could be questioned in the Ishmael/Isaac case due to Ishmael's parentage or circumstances, the Jacob/Esau story leaves no room for such arguments. They were in every outward sense equally 'children of Abraham' and 'children of Isaac,' yet God chose the younger, Jacob, and passed over the elder, Esau.
This sets the stage for Paul's main point: God's sovereign choice operates independently of human lineage or perceived merit.