Besides, she is indeed my sister, the daughter of my father though not the daughter of my mother, and she became my wife.
Genesis 20:12 ESV

This relationship isn't necessarily what it sounds like to modern Westerners. There are several ways that Sarah could have been Abraham's sister, some of which would not have been considered incest at the time, and one of which would be very normal.

*Terah had multiple wives and Sarah was literally his daughter by one of his other wives.
*Sarah was Terah’s granddaughter through one of Abraham’s brothers.
*Sarah was a slave purchased by Terah as a wife for Abraham, in which case Torah says she must be treated like a daughter

I think she most likely was Abraham's half-sister, but such a marriage would not have been a problem yet since the human genome had not deteriorated to the point where it would need to be forbidden.