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’wife; thou shalt not uncover her nakedness.
Read kabbalistic (mystical) interpretation of Leviticus 18:15: and classical mystical sources.