(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 (2Samuel 13:13; 2Samuel 13:16; 2Samuel 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."

Verse 9. - In the third place, incest with a sister is forbidden, and it is specifically stated that under the term "sister" is meant the half-sister, the daughter of thy father, or... thy mother,... born at home, as would naturally be the case if she were the father's daughter, or born abroad, that is, the daughter of the mother by a previous marriage, when she belonged to a different household. Tamar's appeal to Amnon, "I pray thee speak unto the king; for he will not withhold me from thee," exhibits to us the poor woman grasping at any argument which might save her from her half-brother's brutality, and does not indicate that such marriages were, in the time of David, permissible (2 Samuel 13:29). The exact degree of relationship which existed between Abraham and Sarah is not altogether certain (cf. Genesis 20:12 with Genesis 11:29). Ezekiel reckons this sin in the catalogue of the iniquities of Jerusalem (Ezekiel 22:11).

18:1-30 Unlawful marriages and fleshly lusts. - Here is a law against all conformity to the corrupt usages of the heathen. Also laws against incest, against brutal lusts, and barbarous idolatries; and the enforcement of these laws from the ruin of the Canaanites. God here gives moral precepts. Close and constant adherence to God's ordinances is the most effectual preservative from gross sin. The grace of God only will secure us; that grace is to be expected only in the use of the means of grace. Nor does He ever leave any to their hearts' lusts, till they have left him and his services.The nakedness of thy sister,.... To lie with one in so near a relation is exceeding criminal, and for which the law curses a man, Deuteronomy 27:22; and to marry her is not lawful; for though it was necessary for the propagation of mankind that a man should marry his sister, for who else could Cain and Abel marry? yet afterwards, when there was an increase of mankind, and there were people enough remote from each other, it became unlawful for persons in such near ties of consanguinity to marry with each other; though the Egyptians did, in imitation of Isis and Osiris (e), and so the Persians, following the example of Cambyses (f):

the daughter of thy father, or the daughter of thy mother; whether she is a sister both by father and mother's side, or whether only by the fathers side and not the mother's, as Sarah was to Abraham, Genesis 20:12; or only by the mother's side and not the father's:

whether she be born at home or born abroad; not whether born and brought up in his and her father's house, or born and brought up in another place and province; though there were some, as Aben Ezra observes, that so interpreted it, according to the sense of the word in Genesis 50:23; but rather the sense is, as that writer gives it, whether born according to the law of the house of Israel, after espousals and marriage, or without it; that is, whether begotten in lawful marriage or not, whether a legitimate offspring or spurious, born in adultery and whoredom, whether on the father or mother's side; so the Targum of Jonathan, whom thy father begat of another woman, or of thy mother, or whom thy mother bore or brought forth, of thy father, or of another man; and to the same purpose Onkelos:

even their nakedness thou shalt not uncover; neither lie with, or have carnal knowledge of, nor marry one or the other.

(e) Diodor. Sicul. l. 1. p. 23. (f) Herodot. Thalia, sive, l. 3. c. 31.