(12) Thy father's sister.--According to the law which obtained in the time of Christ this prohibition not only extended to the father's half-sisters, but even when they were begotten by the grandfather illegitimately. It is remarkable that Moses himself was the offspring of such an alliance, since his father Amram married his own aunt Jochebed, who was the sister of his father. (See Exodus 6:20.)

Verses 12-15. - Fifthly, incest with a paternal or maternal aunt is forbidden; sixthly, with an aunt by marriage; seventhly, with a daughter-in-law. The last of these finds its place in Ezekiel's catalogue of abominations (Ezekiel 22:11; cf. Genesis 28:18, 26).

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.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.