98b bottom line
The gemoro says that none of the kohanim can do yibum after sofek chalitza of the others, cos a kohen is ossur to a chalutza.
But the gemoro says right at the top of 24a that a kohen can do yibum lechatchila with a sofek chalutza cos its only an issur derabonon??
So here as well, the wife of the sofek brother has had 4 sofek chalitzas and perhaps this yibum from the kohen is the real yibum, so it should be mutar lechatchila??
Avrumi Hersh, London england
1) Rashi (24am DH mid'Rabanan) writes that the Chachamim did not make a Gezeirah to force a Kohen to divorce a Safek Chalutzah. This implies that we are lenient for a Safek Chalutzah only if the Kohen has already married her. The Gemara does not say that a Kohen may do Yibum l'Chatchilah with a Safek Chalutzah.
2) The Chidushei ha'Ramban (24a) writes that the reason why Rashi writes that we are lenient with a Safek Chalutzah only if the Kohen has already married her, and we do not allow him l'Chatchilah to marry her, is because since he performed a major act of Chalitzah in Beis Din we no longer apply the general rule of Safek d'Rabanan l'Kula even l'Chatchilah.
3) The Ramban proves from our Gemara that Rashi is correct. He writes that the reason why, in our Gemara, the Kohanim cannot do Yibum is because Safek Chalutzah is forbidden.
4) Since Chalitzah was done publicly in Beis Din, it looks to everyone that the co-wife is the real Yevamah. If we would allow the Kohen to do Yibum l'Chatchilah with the Safek Chalutzah, it would appear to the world that one is allowed to marry the sister of a Zekukah.
5) The question of the Hafla'ah and the Aruch la'Ner on the Ramban:
a) The Chelkas Mechokek (on Shulchan Aruch EH 6:1) cites Rashi (24a) that l'Chatchilah the Chachamim made a Gezeirah even on a Safek Chalutzah. The Nesivos la'Shoves on Even ha'Ezer 6:1 (by the author of the Hafla'ah and Panim Yafos) cites the proof of the Ramban from our Gemara but writes that in his opinion there is no proof from here. This is because here it is not an equal Safek since most of the women are forbidden as a Chalutzah to a Kohen.
b) The Aruch la'Ner here also writes that he does not understand the proof of the Ramban, since in the case on 24a there are only two Kohanim, so it is a Safek ha'Shakul and we can be lenient. In contrast, in our Sugya there are several brothers so we should follow the majority and say that she is a Vadai Chalutzah.
c) I want to suggest that the Chidushei ha'Rashba on 24a also does not agree with the proof of the Ramban from our Sugya, since he cites Rashi but does not bring a proof from here.
d) At any rate, we still have to understand what the proof of the Ramban is from here. See the Birkas Avraham here.
Kol Tuv,
Dovid Bloom