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."
Cross References (ranked by relevance)
Leviticus 20:12
And if a man lie with his daughter in law, both of them shall surely be put to death: they have wrought confusion; their blood shall be upon them.
Ezekiel 22:11
And one hath committed abomination with his neighbour’s wife; and another hath lewdly defiled his daughter in law; and another in thee hath humbled his sister, his father’s daughter.
Genesis 38:26
And Judah acknowledged them, and said, She hath been more righteous than I; because that I gave her not to Shelah my son. And he knew her again no more.