Adam Clarke on Genesis 12:13

Say, I pray thee, thou art my sister - Abram did not wish his wife to tell a falsehood, but he wished her to suppress a part of the truth. From Gen 20:12, it is evident she was his step-sister, i.e., his sister by his father, but by a different mother. Some suppose Sarai was the daughter of Haran, and consequently the grand-daughter of Terah: this opinion seems to be founded on Gen 11:29, where Iscah is thought to be the same with Sarai, but the supposition has not a sufficiency of probability to support it.