It was bothering me why in the case of the Mishna on 98b where 5 boys got mixed up, does the Gemara say that each one should do a Yibum (not one to all 5 wives) because maybe they all get the right yevama. I recall a story which a Rav once responded ( I think it was Reb Chaim Brisker) that a broken clock is right twice a day. One don't we apply that theory here and have one brother do all 5 yibums and thereby assuring us of one proper yibum?
Sidney Teichman
The Yam Shel Sh'lomo writes that in a case where there were only two
children mixed up we would favor the alternative, i.e. having one be Meyabem both so as to be guaranteed at least one true Yibum.
The Yashresh Ya'akov explains that in the case of the Mishnah where there are five boys mixed up we cannot have one being Meyabem all of the women, for the Gemara on 44a discourages marrying more than four wives. Therefore we favor each one doing one Yibum rather than one taking four and another one taking one, since by doing so we will not be guaranteed a Kiyum.
This also explains why the Mishnah chose a case of five children when the Halachah would have been clear even with fewer.
Dov Zupnik