Vayikra 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).

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.