You mention Eshes Aviv me'Imo, but I think you mean Eshes Aviv me'Aviv (see Rashi). Rashi means that the Mishnah lists Eshes Aviv me'Aviv and says that her Tzarah is Mutar since it cannot happen in a Yibum case. That is, if Reuven, the brother, died without children and Shimon is his brother, there will never be a case in which Reuven's widow is Shimon's Eshes Aviv me'Aviv with children, because if she was once married to Levi (another brother), had children, and then Levi died, she would not had been able to marry Reuven at all, since she is also his Eshes Ach.

All the best,

Reuven Weiner