Thy sister - What was here spoken of was the distinguishing offence of the Egyptians.

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.

By the sister, the daughter of father or mother, we are to understand only the step-or half-sister, who had either the same father or the same mother as the brother had. The clause, "whether born at home or born abroad," does not refer to legitimate or illegitimate birth, but is to be taken as a more precise definition of the words, daughter of thy father or of thy mother, and understood, as Lud. de Dieu supposes, as referring to the half-sister "of the first marriage, whether the father's daughter left by a deceased wife, or the mother's daughter left by a deceased husband," so that the person marrying her would be a son by a second marriage. Sexual intercourse with a half-sister is described as חסד in Leviticus 20:17, and threatened with extermination. This word generally signifies sparing love, favour, grace; but here, as in Proverbs 14:34, it means dishonour, shame, from the Piel חסּד, to dishonour.

The nakedness of thy {e} sister, the daughter of thy father, or daughter of thy mother, whether she be born at home, or born abroad, even their nakedness thou shalt not uncover.

(e) Either by father or mother, born in marriage or otherwise.

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.