Комментарий к Вайикра 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).

אחותך היא, “she is your sister.” Here the Torah speaks of a “sister” who is the result of an official union, whereas previously, in verse 9 it spoke of the result of a casual relationship. The same is true when the Torah spoke of “the daughter of your mother by marriage, all the more so the daughter, i.e. a sister who is the result of carnal relations between either your father or your mother outside the bonds of marriage.