What are the implications of "Achoscha Bas Avicha"?
Rashi: It includes even the daughter of a woman that his father raped.
What are the implications of "Moledes Bayis O Moledes Chutz"?
Rashi: It implies whether his father is permitted to retain the woman's mother, or we tell him to send her away - if for example, she is 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 - even if she is one of the Chayvei K'risus. 2
Ramban #2, Moshav Zekenim 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
Targum Onkelos: It means whether it is a sister whom your father bore from another woman or whom your mother bore from another man.
Targum Yonason: It implies whether she is the daughter of his father from another woman or from his mother, or the daughter of his mother from another man or from his father. 4
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.
Ramban: On whom Kidushin do not take effect. The only exceptions are the daughter of a Shifchah or a Nochris, who goes after her mother and is not considered his sister. 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).
Peirush Yonasan: "Moledes Bayis" is one born to your father or mother from his or her first marriage; "Moledes Chutz" is one born to your father or mother from his or her second marriage. (See also Na'ar Yonasan.)
QUESTIONS ON RASHI
Rashi writes that "Moledes Chutz" is a sister born through a woman forbidden to his father (e.g. due to a Lav) and he must divorce her, Even if she is forbidden through Kares, and she cannot have Kidushin to his father, our verse forbids her!
Ramban: It is even if she is forbidden through Kares.
Mizrachi: Rashi means, whether the sister is a Mamzer (born through Chayavei Kerisus) or e.g. a Nesinah, born through Chayavei Lavim.
Gur Aryeh: Rashi agrees that the verse applies to even if she was born through Chayavei Kerisus. He mentions Mamzeres or Nesinah to illustrate when we tell him to divorce her. If she is Chayavei Kerisus, she is not married to him at all!
