עֶרְוַ֨ת בַּת־אֵ֤שֶׁת אָבִ֙יךָ֙ מוֹלֶ֣דֶת אָבִ֔יךָ אֲחוֹתְךָ֖ הִ֑וא לֹ֥א תְגַלֶּ֖ה עֶרְוָתָֽהּ׃ (ס)
The nakedness of thy father’s wife’s daughter, begotten of thy father, she is thy sister, thou shalt not uncover her nakedness.
Read kabbalistic (mystical) interpretation of Leviticus 18:11: and classical mystical sources.