The Mishna (2a) listed 15 women who – in the event that their husband passes on with no offspring – cannot become a yevama and marry his surviving brother, due to the fact that they are closely related to him (e.g. if the brother who passed away had married his niece, her father cannot perform yibum with her). Moreover, in the event that her first husband was married to another woman at the same time (a tzarah in the language of the Mishna), her fellow-wife will not become a yevama either.
The Mishna concludes with a limitation to this rule. If the woman who is related to the yavam dies, is divorced or is discovered to be an aylonit, she is removed from the picture and the remaining wife will become a yevama, should her husband pass away without children.
An aylonit is a woman whose physical makeup will not allow her to have children (the term stems from the word ayil – a male ram – alluding to her lack of femininity). An aylonit does not become a yevama because the point of yibum, as clearly stated in the Torah, is to bear children in order to keep the dead brother “alive” (see 25:6, which states that the first child to be born will be named for the late brother).
The Gemara on our daf quotes Rav Assi who rules that the tzarah of an aylonit cannot become a yevama, and, in fact, is forbidden to marry any of the brothers. Apparently Rav Assi believes that we must distinguish between an aylonit who the original husband married with full knowledge of her condition and one who was only discovered to be an aylonit later on. The latter case is the one discussed in the Mishna. Since the husband was unaware that his wife was an aylonit we can assume that the marriage was to be annulled since it took place under circumstances that he would not have agreed to. When the husband knew that she was an aylonit and accepted that situation, the marriage certainly is a proper one, and neither she nor her tzarah will become yevamot.