You have to say the Gemara in Bava Batra is thinking there is a hazaka for a brother and still she is permitted to marry (without yibum or halitza) because he is also saying he has a son. This is because in Kidushin the whole difference between the Mishna and the Braita over there is when there is a hazaka that there is a brother. The Gemara over in Kidushin clearly say that is the only difference and that is the reason R Nathan forbids her. So then what do you do with the Mishna in Bava Batara that is the exact same law? You have to say in Bava Batra he is also saying he has a son.
[I did not get a chance to do any learning today but it occurs to me that this might be the very reason the Ramban (Nahmanides) is disagreeing with the approach of the Baal Hameor and Rashi.
Besides one other reason he gives that כיוון שהגיד שוב אינו מגיד. [Once he says something and believed he is no longer believed to change his mind. But besides that I can imagine the Ramban (Nahmanides) also is thinking that it makes no sense to put in an extra condition in the Mishna in Bava Batra--especially when over there it is clearly divided into two case. One when he says he has a son and the other when he claim to have a brother.
[I did not get a chance to do any learning today but it occurs to me that this might be the very reason the Ramban (Nahmanides) is disagreeing with the approach of the Baal Hameor and Rashi.
Besides one other reason he gives that כיוון שהגיד שוב אינו מגיד. [Once he says something and believed he is no longer believed to change his mind. But besides that I can imagine the Ramban (Nahmanides) also is thinking that it makes no sense to put in an extra condition in the Mishna in Bava Batra--especially when over there it is clearly divided into two case. One when he says he has a son and the other when he claim to have a brother.
