Levililer 18:11

ערות בת אשת אביך THE NAKEDNESS OF THY FATHER'S WIFE'S DAUGHTER… [THOU SHALT NOT UNCOVER] — This teaches us that one is not liable to excision for a union with one's sister (on his father’s side) who was born of a maid-servant or a non-Jewess. It is for this reason that Scripture instead of simply saying "thy sister" uses the circumlocution "the daughter of thy father's wife" i. e. a woman who is fitted for Kiddushin (one with whom a marriage may be legitimately entered into, which is not the case with a שפחה ונכרית) (Yevamot 22b).

The nakedness of thy father’s wife’s daughter, begotten of thy father, she is thy sister, thou shalt not uncover her nakedness.

עֶרְוַ֨ת בַּת־אֵ֤שֶׁת אָבִ֙יךָ֙ מוֹלֶ֣דֶת אָבִ֔יךָ אֲחוֹתְךָ֖ הִ֑וא לֹ֥א תְגַלֶּ֖ה עֶרְוָתָֽהּ׃ {ס}