Charles Ellicott Commentary Leviticus 18:9

"The nakedness of thy sister, the daughter of thy father, or the daughter of thy mother, whether born at home, or born abroad, even their nakedness thou shalt not uncover." — Leviticus 18:9 (ASV)

The nakedness of your sister. —The fact that Adam married bone of his bone and flesh of his flesh, and that his sons married their own sisters, encouraged the ancient Hebrew to imitate their example. Therefore, we find Abraham, the father of the faithful, married his half-sister (Genesis 20:12). The same practice was common among other nations of antiquity. Thus, the Athenians married their half-sisters by their father’s side, and the Spartans married half-sisters by the same mother, while the Assyrians and Egyptians married full sisters.

Though nothing can be more explicit than the law here laid down, and though its transgression is denounced as an accursed and impious crime, punishable by capital punishment (Deuteronomy 27:22), yet from the narrative of Amnon and his sister Tamar, and especially from the touching and melancholy remark of the outraged sister (2 Samuel 13:13; 2 Samuel 13:16; 2 Samuel 13:20), it is evident that the practice of the primitive parents of the human race and the example of the father of the Hebrew nation continued to be followed in spite of this law .

Born at home or born abroad. —Literally, the birth, or offspring of the house or the birth, or offspring from abroad. According to the administrators of the law during the Second Temple period, the meaning of this precept is to forbid sexual relations between a brother and a sister, whether the sister is born in wedlock (which is meant by born at home) or whether she is illegitimate (which is meant by birth or offspring from abroad). Therefore, the ancient Chaldee Version renders this clause as: “whom your father begot by another woman or by your mother, or whom your mother bore by your father or by another man.”