Leviticus 18:9

The nakedness of thy 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. Hence we find Abraham, the father of the faithful, married his half-sister (Genesis 20:12). The same practice obtained amongst 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, whilst the Assyrians and Egyptians married full sisters. Though nothing can be more explicit than the law here laid down, and though the transgression of it is denounced as an accursed and impious crime, to be visited with capital punishment (see Leviticus 20:17; 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. (Comp. Ezekiel 22:11.)

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, the import of this precept is to forbid commerce 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. Hence the ancient Chaldee Version of this clause, “whom thy father begot of another woman or of thy mother, or whom thy mother brought forth by thy father or by another man.”