Charles Ellicott Commentary Leviticus 18:11

"The nakedness of thy father`s wife`s daughter, begotten of thy father, she is thy sister, thou shalt not uncover her nakedness." — Leviticus 18:11 (ASV)

Your father’s wife’s daughter. —If this clause stood alone, it would denote the daughter of a man’s stepmother by another or previous husband, since “father’s wife” in Hebrew always denotes stepmother (see Leviticus 18:8, Leviticus 20:11; Deuteronomy 23:1,Deuteronomy 26:20). In this case, the man and the young woman, though not blood relations at all, would be forbidden to each other because the young woman’s mother had married the man’s father. It would thus differ from Leviticus 18:9, where the young woman is a half-sister either by the same father or the same mother.

Begotten of your father. —Literally, the birth, or offspring of your father . That is, though the daughter of the stepmother, she is begotten by the same father and is therefore his half-sister on the father’s side. This is exactly the same case already prohibited in the first clause of Leviticus 18:9.

Therefore, to avoid a senseless repetition of the same prohibition, we must either regard this clause as having crept into the text from a marginal gloss, or we must correct the first letter of the disjunctive particle in Leviticus 18:9, which would make it the nakedness of thy sister, the daughter of thy father and the daughter of thy mother. Accordingly, Leviticus 18:9 prohibits marriage with a full sister, while the verse before us forbids it with a half-sister. The latter is the more probable, since intermarriage between entire stepbrother and stepsister has always been, and still is, legitimate among the Jews.