Matthew Poole and continuators on 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 .

My near Kinswoman; even as Lot upon the same account is called Abrahams Brother, Gen. 13. 8.

(i. e.) The Granddaughter: For Grand-children are commonly called the Sons and Daughters of their Grand-parents, as Gen. 31. 28. Exod. 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 Terahs Daughter and Abrahams Sister. See Gen. 11. 29.

Because Haran was Abrahams Brother onely by the Fathers side, Terah who 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, Levit. 18. Besides among all Nations the Mothers side was more regarded than the Fathers in all prohibitions of Marriage.