Question: two "nochrim" are married to two sisters. One has one brother, the
other two. When the two men die we say that the single brother is "HOLETZ" , because of a
doubt of "ahot zkukato" , and the two brothers - one of them is HOLETZ and the other
is MEYABEM - since he has no doubt. Why couldn't we say the opposite and
start from the two brothers and say that after the second brother is meyabem - the
remaining sister falls to the "one brother" and he could meyabem her.?
In the case that you refer to -- when two men (Reuven and Shimon) were Mekadesh two sisters, and neither of the men knew which sister he had married, and then the two men died, and one man (Reuven) had one brother and the other (Shimon) had two brothers -- it is not possible for either of the brothers of Shimon to do Yibum until the brother of Reuven first does Chalitzah. This is because the woman with whom the second brother of the Shimon intends to do Yibum (even after the first brother of Shimon did Chalitzah with one of the sisters) might actually be Reuven's Yevamah, and since Reuven's brother has not yet done Chalitzah or Yibum with her, this second brother would be marrying a Yevamah who did not first do Yibum (and she is Asur as a "Yevamah l'Shuk").