Charles Ellicott Commentary Leviticus 18:15

"Thou shalt not uncover the nakedness of thy daughter-in-law: she is thy son`s wife; thou shalt not uncover her nakedness." — Leviticus 18:15 (ASV)

Your 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 also 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 . Other nations regarded such alliances with the same abhorrence (see Koran 4:27).