Genesis 20:12

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 between the identity of the father and the diversity of the mother would in her case be unmeaning. Sarah was apparently Abraham’s half-sister, being Terah’s daughter by another wife; and we gather from her calling her child Sarai — that is, princely (see Genesis 17:15) — that she was not a concubine, but belonged to some noble race.