Leviticus 20:19

'וערות אחות אמך וגו‎‎‎ [AND THOU SHALT NOT UNCOVER] THE NAKEDNESS OF THY MOTHERS SISTER [NOR OF THY FATHER’S SISTER] — Scripture repeats the prohibition regarding these (although they had been mentioned already in Leviticus 18:12, 13) to tell us that one is prohibited in respect to them, both in regard to one's father's or mother's sister on their father's side, and in regard to their sisters on their mother's side. But as regards a marriage being forbidden with the wife of one's father’s brother (cf. Leviticus 18:14) the prohibition (i.e. the איסור ערוה מדאורייתא) applies only to the wife of the brother of one's father when he is the father’s brother on the father’s side (i. e. when both have one father) (cf. Yevamot 54b).

וְעֶרְוַ֨ת אֲח֧וֹת אִמְּךָ֛ וַאֲח֥וֹת אָבִ֖יךָ לֹ֣א תְגַלֵּ֑ה כִּ֧י אֶת־שְׁאֵר֛וֹ הֶעֱרָ֖ה עֲוֺנָ֥ם יִשָּֽׂאוּ׃

And thou shalt not uncover the nakedness of thy mother’s sister, nor of thy father’s sister: for he uncovers his near kin: they shall bear their iniquity.