(15) Thy daughter-in-law.--The legislators in the time of Christ defined this prohibition as applicable not only to cases where marriage between them had actually been consummated, but to cases where the maiden had only been espoused, or when the daughter-in-law had been divorced by the son, or had become a widow. For an offence of this kind both parties were punished with death. (See Leviticus 20:12.) Other nations regarded such alliances with the same abhorrence. (See Koran, 4:27.)

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 shall not uncover the nakedness of thy daughter in law,.... Shall not he with her in his son's lifetime, or marry her after his death:

she is that son's wife; and so one flesh with him, and who is of the same flesh and blood with his father, and therefore the nearness of the relation forbids such incestuous copulation or marriage:

thou shall not uncover her nakedness; or have carnal knowledge of her, whether in the life or after the death of his son, even then marriage with her is not lawful.