What are the implications of "Achoscha Bas Avicha"?
Rashi: It implies even if she is the daughter of the woman that his father raped.
Targum Onkelos and Targum Yonasan: It means whether it is a sister whom your father bore from another woman or whom your mother bore from another man. 1 Yonasan: It implies whether she is the daughter of his father or his mother from their first marriage or from their second marriage.
Targum Yonasan adds 'whom your father bore from your mother' and 'whom your mother bore from your father' (See Na'ar Yonasan).
And what are the implications of "Moledes Bayis O Moledes Chutz"?
Rashi: This implies whether his father is permitted to retain the woman's mother or we tell him to send her away, if she is for example a Mamzeres or a Nesinah. 1
Ramban #1 (citing Yevamos, 23a): It implies whether his father is permitted to retain the woman's mother or we tell him to send her away - because she is one of the Chayvei Kerisus (or Chayvei Misos Beis-Din). 2
Ramban #2 and Seforno: With reference to the daughter of his mother, it implies that his sister is forbidden, irrespective of whether she is born to her from a man to whom she is married or from a man with whom she had illicit relations. 3
Whom his father is forbidden to retain because she is one of the Chayvei La'avin - even though the Kidushin was effective. See Ramban's objection to this explanation.
On whom Kidushin do not take effect. The only exceptions are the daughter of a Shifchah or a Nochris, whose status she adopts (Ramban). Refer to 18:11:1:1.
Which we might otherwise have thought he (her brother) is not committed to treat with the honor of a sister (Ramban).