Why is the case of the near-simultaneous deaths in Y'vamos chapter 7?

Question

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?

Answer 1

In each of the three cases we have a Hazka Kamayita.

For the first two cases, (safek 9 year old and safek puberty) Meyikar Haddin, She can eat teruma (The safek 9 year old is still not 9 old, and his bia cannot make a bat Kohen pesula. The Israel non Cohen is still not pubery, the bat Kohen can continue to eat Teruma). However the Mishna rules Chumra against the Chazaka.

The third case also when husband dead, the erva wife is bechezkat Haya (see Tosfot 31a "nafal" for deep explanation). However the yebama is prohibited lashuk against the Chazaka.

Gemara in Daf 31a objects against such a Chumra that in a similar case (safek kiddushin for erva which is Tsara of yebama) the yebama is allowed to marry with everyone. Gemara answers that in frequent cases, we don't make chumra against Chazaka because it's a chumra that leads to Kula (חומרא דאתי לידי קולא) If she needs Chalitsa, people may allow Ybum, and lead to Karet Meyikar Hadin. But in not frequent cases (לא שכיחי), we are not afraid about Chumra deAte lidey kula.

So the group of 3 cases is homogeny.

Here we have three not frequent cases and we can make chumra against Chazaka.

In safek 9 year old and is Kohen, people can validate his Ybum and she will eat teruma which is mita bidey shamayim against Chazaka. Or When he is Kohen makes Kiddushin and is safek puberty, and is dead, his brother will make ybum and she will eat teruma against Chazaka (following the Chazaka kiddushin aren't kiddushin, and therefor ybum is not ybum, and she is zara who eat teruma).

All this cases are rare and we are not afraid of chumra deate lidey kula, חומרא דאתי לידי קולא

This is the common reason of the three dinim.

Even if we don't know Gemara 31a, we can understand this from merarshim

TYT

ספק בן תשע שינים וכו': ולא מוקמינן ליה אחזקה ‏

Chumra against Chazaka.

ROB:

ואיידי דאיירי בספקי לחומרא תניא להא בהדיהו

Since they are talking about safek lechumra, they taught it with them. [we need to add that there is a Chashash of חומרא דאתי לידי קולא and that these cases are not frequent. And since they are not frequent, this Chashash is negligible].