Charles Ellicott Commentary Genesis 20:12

"And moreover she is indeed my sister, the daughter of my father, but not the daughter of my mother; and she became my wife:" — Genesis 20:12 (ASV)

Not the daughter of my mother. —This disproves the notion that Sarah was the same as Iscah (Genesis 11:29). For, as Iscah was Terah’s granddaughter, the distinction of having the same father but a different mother would, in her case, be meaningless. Sarah was apparently Abraham’s half-sister, being Terah’s daughter by another wife. Furthermore, we gather from her child being called Sarai—that is, princely ()—that she was not a concubine but belonged to some noble race.