The Gemara is trying to figure out what types of Ervas are included in the list that exempt their Tzaros from Yibum. There are those that include cases that not everyone agrees to. Furthermore, there are those that include cases that comes out from illegal relations.
Tosfos asks: if so, why don't we include the case where someone divorces his wife on condition that she doesn't marry Ruvain. Then she married Ruvain's brother and he dies. So not only can't Ruvain do Yibum on this woman, since to him she's an Ervah i.e. a married woman to the first husband, but also he can't do Yibum her Tzara either.
Although the Rabanan hold you cannot divorce your wife unless you allow her to marry everyone. However, since we count even cases where there are arguments, since R' Eliezer holds this type of Get to work we can still count it.
And even though R' Eliezer agrees that she shouldn't marry anyone until Ruvain dies. Since she might become widowed or divorced to the second husband she might marry Reuvain and make her divorce retroactively null. However, we are also listing cases if they get married although they're not allowed to.
Tosfos answers: we learn that an Ervah is exempt from Yibum from a wife's sister. This woman that got divorced on condition is not similar to a wife's sister. Although she can't marry Ruvain, she can have out of wedlock relations with him (since the condition was only not to marry him.) Even if the husband divorced her on condition not to have relations with Ruvain, it's still not similar, since he can give her Kiddushin (but there is no Kiddushin that can take affect by a wife's sister)
Even if the condition is not to marry or have relations, it's still not similar, since she's not a married lady when Ruvain's brother dies. Only if Ruvain does Yibum to her does she retroactively become a married lady. This is not similar to a wife's sister who has that status at the death of his brother.
Even if we consider her immediately the Erva of a married lady, since that is the reason Ruvain cannot do Yibum to her, it is still not similar. Because here if Ruvain would do Yibum to her, then it would retroactively make her not married to his brother and therefore her Tzara was retroactively not her Tzara and therefore permitted to Ruvain.
