Parashas Acharei Mos · Passuk 18:11
Chabad.org — English translation + Rashi
Torah Temima points to Yevamos 22b.
English: “Daughter of your father’s wife” — excludes a sister born of a maidservant or a gentile woman, with whom the father had no marriage bond.
Exclusion of non-marital unions
This particular count requires that the mother was the father’s wife in the full sense of ishut.