The succession of rulers in Genesis indicates that first-born sons of sister brides were in line to rule after their fathers. So Isaac was in line to rule after Abraham, since Sarah was Abraham's half-sister. The first-born sons of cousin brides did not rule over their fathers’ territories. Instead they ascended to the throne of their maternal grandfathers. So Joktan, Abraham's first-born by his cousin bride Keturah, would succeed to the throne of Joktan, Keturah's father.
After years of studying descent systems, Claude Levi-Strauss noted that in patrilineal systems mother and child belong to different clans. In the case of Abraham’s ancestors, the cousin bride belongs to her husband’s house while her first-born son belongs to her father’s house. This is confirmed in Genesis 10:22, where we find that Nimrod’s first-born son by a daughter of Asshur is called a “son” of Shem.
Nimrod = Asshur’s daughter
Asshur the Younger (“son” of Shem)