Leviticus 18:15

אשת בנך [THE NAKEDNESS OF THY DAUGHTER-IN-LAW THOU SHALT NOT UNCOVER; SHE IS] THY SON'S WIFE — By using the apparently redundant words: “she is thy son's wife" Scripture says, as it were: I command this about such a woman only when thy son has a legal matrimonial relation (אישות) to her, thus excluding a woman who has been raped by him (אנוסה) or a Canaanitish maid-servant, or a non-Jewess (cf. Sifra, Kedoshim, Chapter 10 10)).

עֶרְוַ֥ת כַּלָּֽתְךָ֖ לֹ֣א תְגַלֵּ֑ה אֵ֤שֶׁת בִּנְךָ֙ הִ֔וא לֹ֥א תְגַלֶּ֖ה עֶרְוָתָֽהּ׃ {ס}        

Thou shalt not uncover the nakedness of thy daughter in law: she is thy son’s wife; thou shalt not uncover her nakedness.