Rashi on Leviticus 18:9/רש"י על ויקרא י״ח:ט׳

1בת אביך [THE NAKEDNESS OF THY SISTER] THE DAUGHTER OF THY FATHER [OR THE DAUGHTER OF THY MOTHER… THOU SHALT NOT UNCOVER] — Also the daughter of a woman raped by one's father is implied by the term מולדת בית או מולדת חוץ.

2.בת — WHETHER SHE BE BORN IN THE HOUSE OR WHETHER SHE BE BORN OUTSIDE THE HOUSE — This means, thy sister, thy father's daughter, whether according to the law we can say to thy father, "Keep her mother as your wife" or whether we have to say to him, "Divorce her mother", as, for instance, when she is a ממזרת (the issue of a union forbidden in the Torah under the penalty of כרת) or a נתינה (the daughter of a Gibeonite woman, cf. Joshua 9:27, forbidden under the penalty of מלקות) (Yevamot 23a).