עֶרְוַ֨ת אֲחֽוֹתְךָ֤ בַת־אָבִ֙יךָ֙ א֣וֹ בַת־אִמֶּ֔ךָ מוֹלֶ֣דֶת בַּ֔יִת א֖וֹ מוֹלֶ֣דֶת ח֑וּץ לֹ֥א תְגַלֶּ֖ה עֶרְוָתָֽן׃ (ס)
The nakedness of thy sister, the daughter of thy father, or the daughter of thy mother, whether born at home, or born abroad, even their nakedness thou shalt not uncover. .
Read kabbalistic (mystical) interpretation of Leviticus 18:9: and classical mystical sources.