עֶרְוַ֥ת אֲחוֹת־אָבִ֖יךָ לֹ֣א תְגַלֵּ֑ה שְׁאֵ֥ר אָבִ֖יךָ הִֽוא׃ (ס)
Nagości siostry ojca twojego nie odkrywaj: pokrewna to ojca twojego.
Sefer HaChinukh
To not reveal the nakedness of the sister of the father: To not have intercourse with the sister of the father, as it is stated (Leviticus 18:12), "The nakedness of the sister of your father you shall not reveal, etc."