John Gill 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
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^F5, and so the Persians, following the example of Cambyses^F6.

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

[whether she is] born at home or born abroad; not whether born and brought up in his and your 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 your father begat of another woman, or of your mother, or whom your mother bore or brought forth, of your father, or of another man; and to the same purpose Onkelos.

[even] their nakedness you shall not uncover; neither lie with, or have carnal knowledge of, nor marry one or the other.

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FOOTNOTES:

• F5: Diodor. Sicul. l. 1. p. 23.
• F6: Herodot. Thalia, sive, l. 3. c. 31.