The seventh chapter of Mishayos Y'vamos discusses under what circumstances someone related to a kohen can eat t'ruma, depending on their relation and on what other relatives are alive. But in the middle of the chapter (the end of mishna 4) is a seemingly unrelated ruling, that if we don't know who died first, a (childless) husband or his wife, and the dead wife is an erva-relative of the yavam's, then the yavam must perform chalitza with the other wife: he must do something in case the wife died first and the remaining wife needs chalitza, but cannot do yibum in case his brother died first and the wives need nothing because one is the brother's relative.
Why is that ruling in this chapter? It has seemingly nothing to do with the rest of the chapter.
I do see the Rav's explanation, that "because it's dealing with doubtful cases in which we rule stringently, it taught another with them", but find it unsatisfying: for one thing, there are many such cases it could have added, and this explanation doesn't say why this one was chosen; for another, this seems a weak reason to add any such case. Does anyone know of another explanation?