Sarah was his sister.
Genesis 19:35
“But indeed she is truly my sister. She is the daughter of my father, but not my mother; and she became my wife.
“And it came to pass, when god caused me to wander from my fathers house, that I said to her, ‘This is your kindness that you should do for me: in every place wherever we go, say of me, “He is my brother”
Damn, that’s wild.