She is my sister, my near kinswoman; even as Lot upon the same account is called Abraham's brother, Genesis 13:8. She is the daughter of my father, i.e. the granddaughter; for grandchildren are commonly called the sons and daughters of their grandparents, as Genesis 31:28 Exodus 2:18. And besides, her father Haran dying before her grandfather, she was left more immediately under his care and education, and therefore was more peculiarly reputed Terah's daughter, and Abraham's sister. See Genesis 11:29. But not the daughter of my mother, because Haran was Abraham's brother only by the father's side; for Terah had Haran by another wife. How could Abraham marry one so near of kin to him? Answ. There were larger allowances for marriages in those times, as it was convenient there should be; neither had God as yet given those prohibitions, Leviticus 18:1-30. Besides, among all nations, the mother's side was more regarded than the father's in all prohibitions of marriage.
Related Commentaries of Genesis 20:12
Genesis 20:12
12 And yet indeed she is my sister; she is the daughter of my father, but not the daughter of my mother; and she became my wife.