Leviticus 18:9

The nakedness of thy sister, the daughter of thy father, or the daughter of thy mother, (or of her) whom, thy father begat by another wife, or of thy mother, whom thy mother bare by thy father or by another husband, thou shalt not dishonour.

born of the house i.e., born according to the ways of the House of Israel: after betrothal and wedlock.

born outside i.e., born outside of wedlock. Other people interpret born in the sense of “were brought up under Joseph’s guidance” [Genesis 50:23] — she is forbidden to you whether she grew up with you in the same house, or whether she lived in a different town or country and grew up outside of your father’s house. Scripture then proceeds to mention his granddaughter.

THE NAKEDNESS OF THY SISTER, THE DAUGHTER OF THY FATHER, OR THE DAUGHTER OF THY MOTHER, WHETHER BORN AT HOME, OR BORN ABROAD, EVEN THEIR NAKEDNESS THOU SHALT NOT UNCOVER. “Whether born at home, or born abroad — this means whether [the court] says to your father, ‘Keep her mother as your wife,’ or whether it has to say to him, ‘Divorce her mother,’ as for instance when she [i.e., the mother] is a mamzereth or a nethinah” [a mamzereth is the female product from one of the forbidden sexual relationships in this section, and a nethinah is a female descendant of the Gibeonites — relationships with these women being in both cases punishable by whipping]. Thus the language of Rashi. But it is not correct, since this interpretation leads to the conclusion that a sister that is born from [a woman who is herself one of] the relations forbidden [to his father],, the penalty for which is excision [and not mere whipping], it would follow that the child born from that union is not included under the terms of this prohibition; since by stating these two examples — that the mother was either a mamzereth or a nethinah, with whom, in both of which cases, sexual intercourse is punishable only with whipping — Rashi thereby seems to exclude from this verse the case where the mother was one of the forbidden relations, where the penalty is excision! This conclusion is obviously incorrect. is not included in this prohibition [against marrying one’s sister. For since Rashi explained the expressions whether born at home, or born abroad as referring to the mother, whether she is a lawful wife or a mamzereth or nethinah, both of whom are forbidden to the father only by punishment of whipping, and not by excision, unlike all forbidden relations, then the illogical result follows that if the mother were one of those relations forbidden to his father by punishment of excision, then the son’s sister born from that union would not be included in this prohibition]! And yet all [sisters] are forbidden by excision [i.e., regardless of whether the sister was born of a union itself permitted, or forbidden by whipping only, or by excision], unless she be born of a bondwoman or a Cuthean woman [in which case she is not considered his sister at all]!
And I wonder at the Rabbi [Rashi]! For in the Gemara of Tractate Yebamoth in the second chapter it is clearly stated: “Perhaps I can say that this …

מולדת בית או מולדת חוץ; both legitimate or illegitimate, both unmarried. This appears to me to be the plain meaning of the text.