Charles Ellicott Commentary Genesis 11:29

"And Abram and Nahor took them wives: The name of Abram`s wife was Sarai; and the name of Nahor`s wife, Milcah, the daughter of Haran, the father of Milcah, and the father of Iscah." — Genesis 11:29 (ASV)

Iscah. —Not the same as Sarai, for we learn in Genesis 20:12 that she was Abraham’s half-sister—that is, a daughter of Terah by another wife. Nor was she Lot’s wife, as Ewald supposed, for she was his full sister. Marriages between near relatives seem to have been allowed at this time, and were perhaps even common for religious reasons (Genesis 28:1–2), but not marriages between those actually by the same mother. Thus Abraham takes his half-sister to wife, and Nahor his niece. Iscah, like Naamah (Genesis 4:22), was probably eminent in her time, but for reasons not recorded.