Thy father's sister - The instance of Amram and Jochebed Exodus 6:20 seems to show that marriage with an aunt was not considered wrong by the Israelites when they were in Egypt.

Thou shalt not uncover the nakedness of thy father's sister,.... His aunt by his father's side, an instance of which we have in Amram, Exodus 6:20; and Maimonides says (i), an aunt was forbidden whether she was a father's sister in lawful wedlock or in fornication:

she is thy father's near kinswoman; or, the rest of thy father (k); the residue of his flesh, one of the same flesh and blood with him; wherefore, as he could not marry her himself, so his son likewise was too near akin to enter into such a relation with her.

(i) Hilchot Issure Biah, c. 2. sect. 5. (k) "reliquiarum patris tui", Tigurine version.

Marriage or conjugal intercourse with the sister of either father or mother (i.e., with either the paternal or maternal aunt) was prohibited, because she was the blood-relation of the father or mother. שׁאר equals בּשׂר שׁאר (Leviticus 18:6, as in Leviticus 20:19; Leviticus 21:2; Numbers 27:11), hence שׁארה, blood-relationship (Leviticus 18:17).

Thou shalt not uncover the nakedness of thy father's sister: she is thy father's near kinswoman.

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.