עֶרְוַ֥ת אֵֽשֶׁת־אָחִ֖יךָ לֹ֣א תְגַלֵּ֑ה עֶרְוַ֥ת אָחִ֖יךָ הִֽוא׃ (ס)
Thou shalt not uncover the nakedness of thy brother’s wife: it is thy brother’s nakedness.
Read kabbalistic (mystical) interpretation of Leviticus 18:16: and classical mystical sources.