עֶרְוַ֥ת אֵֽשֶׁת־אָבִ֖יךָ לֹ֣א תְגַלֵּ֑ה עֶרְוַ֥ת אָבִ֖יךָ הִֽוא׃ (ס)
The nakedness of thy father’s wife shalt thou not uncover: it is thy father’s nakedness.
Read kabbalistic (mystical) interpretation of Leviticus 18:8: and classical mystical sources.