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. Thou shalt not uncover the nakedness of thy daughter in law: she is thy son’s wife; thou shalt not uncover her nakedness.
Within the sequence of Leviticus 18:6-19, this command stands as one of the boundaries God established to guard the moral purity of His people against sexual conduct that violates those limits. It specifically forbids a man to have sexual relations with his daughter-in-law, on the ground that she is his son's wife. The prohibition extends in principle to improper sexual familiarity between men and women generally. Such conduct is a crime and a sin that destroys moral purity.