Rashi on Leviticus 18:11/רש"י על ויקרא י״ח:י״א

1ערות 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).