Explanation #1 (Rashi): The Yavam is Vadai, for he knows what is his Ko'ach (right to inherit), and he is Muchzak in the property.

Implied question: Why is this different than the case of Rav Mesharshiya above? (After the Yavam (Shimon) died, they came to inherit the property of his brother who died before him (Reuven). The Safek says "I am Reuven's son. I inherit him alone!" Shimon's sons say "you are our brother. You have only a portion with us! All are considered Safek.)

Answer: There, even though the Safek surely owns a share of the property in any case, even so, he is not considered Vadai, for he does not know what is his Ko'ach (if he is Reuven's son or nephew).

Inference: We call "Vadai" one who Vadai receives a share of the property.

Question: In Bava Basra (158b), regarding the Mishnah of a house that fell on him and his mother, both of them (Beis Shamai and Beis Hillel) agree that they divide;

The Rashbam explains that the mother's heirs, i.e. her paternal brothers, divide with the son's heirs, i.e. his brothers from his father Reuven;

He did not want to explain that the mother's heirs are her children, i.e. the son's maternal brothers, e.g. Binyamin, for if so Binyamin would be Vadai and Reuven Safek, and a Safek cannot take from a Vadai.

Rejection #1: It is totally unreasonable that because he is Vadai in part of the property, he is Vadai in all of it!

Rejection #2: We say there, and bring below, that if a house fell on Reuven and his father, or on Reuven and one whom he inherits, and there is [a lien] on Reuven to pay a Kesuvah or creditor [and his property did not suffice. The widow or creditor says that the father died first, so Reuven inherited him, so they may collect from the father's property.]

We conclude that the father's heirs are Reuven's brothers;

Inference: Even though the father has other sons, they are not considered Vadai.

Rejection #3: In Bava Kama (9a), when brothers divided [an inheritance] and a creditor took the portion of one of them (Levi), Rav Asi says that Levi takes a quarter [of what was taken from him] in land or money [from his brother];

This is because he is unsure whether they are like heirs or buyers. (If we say Ein Breirah, we cannot know that a brother received his proper inheritance. It is as if each sold his true share in exchange for the share he took.)

We do not say that he does not receive anything, for the Muchzak (his brother) is Vadai in part of the property!

Rejection #4: One two are holding a Talis, and one of them says "all of it is mine", and the other says "half of it is mine" (Bava Metzi'a 2a, we do not say that the latter gets nothing, for the former is Vadai in part of it)!

Explanation #2: Rather, the Yavam is called Vadai because he is a Vadai heir of the grandfather. The Safek is unsure whether he inherits him at all;

There, when the house fell on him and on his mother, Binyamin is not a Vadai heir in the entire other half. If Rachel (the mother of Yosef and Binyamin) died before Yosef, (Yosef inherited half her property, and) also Reuven (their paternal brother) inherits him.

Similarly, when the house fell on him and on his father, if his father died first, the [son's] creditor "inherits" [half], and not [the father's] other son.

Reference: In Bava Basra (158a), Beis Shamai and Beis Hillel argue similarly about a house that fell on him and on his wife. The Gemara says "in whose Chazakah?"

I.e. Beis Hillel say "the property is in its Chazakah", i.e. Nichsei Tzon Barzel (property that she brought from her father's house. It is appraised, and her husband must return its value when she leaves him). In whose Chazakah is it?

Citation (158a) Answers: R. Elazar says, it is in the Chazakah of the woman's heirs. R. Yochanan says, it is in the Chazakah of the husband's heirs. Reish Lakish says, they divide.

Explanation #1 (Rashi, Kesuvos 80b): Just like they argue there, they argue also about our Mishnah here.

Rebuttal (R. Tam): Here is different than there! Here, Beis Shamai do not argue about the [primary] Kesuvah, only about the Nichsei Melug;

The Gemara says that the Kesuvah was omitted, since it taught "the husband's heirs divide with the father's heirs...", but there it taught Stam "they divide", which implies that they argue also about the Kesuvah.

Explanation: This is because there, if the husband died first, it is as if his wife collected her Kesuvah;

However, here, she is Zekukah l'Yavam, she does not receive anything from her husband's property until after Chalitzah, for the Kesuvah is not collectable in [the husband's] lifetime.

Possibility: Perhaps all the Amora'im agree that here, the Nichsei Tzon Barzel are in the Chazakah of the husband's heirs.

Implied question: If so, why didn't the Mishnah teach it together with the Kesuvah?

Answer: This was to give the reason, I.e. they are in the Chazakah of the one [Muchzak] until now, i.e. the husband.

However, it is more obvious that the Kesuvah is in the husband's Chazakah than Nichsei Tzon Barzel, which she brought from her father's house and they were appraised to her husband.

Support: We must say so according to the opinion there that they are in the Chazakah of the husband's heirs.

Question: According to the one who says in the Chazakah of the wife's heirs, why is it different when the house fell on him and his mother?

Then, Beis Hillel agree that they divide. We do not establish them in the Chazakah of the mother's heirs!

Answer #1: Perhaps that Tana [who discusses when it fell on a man and his wife] holds that the inheritance of those who inherit the husband is mid'Rabanan (i.e. a husband inherits his wife only mid'Rabanan).

Answer #2: Even if he holds that it is mid'Oraisa, since it is not due to kinship, we establish them more in the woman's Chazakah.

Answer #3: We do not establish them in the Chazakah of the mother's heirs, because it is normal that the mother dies first;

In Bava Basra (108a), it is considered a punishment when the son dies first. Therefore, we do not establish them in the mother's Chazakah.

Question: How do the woman's heirs collect the Kesuvah? We hold like Rav and Shmuel, who say (Shevu'os 47a) that one does not bequeath to his sons [money that he can collect only through] a Shevu'ah!

Partial Answer #1: This is not difficult for Beis Shamai, who hold that a document destined to be collected, it is as if it was collected. It is as if they are Muchzakim in the money;

We challenge Rav and Shmuel in Shevu'os from several Beraisos, and answer that the Beraisos are like Beis Shamai.

Partial Answer #2: This is not difficult for R. Elazar, who says (Bava Basra 158a) that it is in the Chazakah of the woman's heirs. He explicitly argues in Shevu'os (48a) with Rav and Shmuel.

Summation of question: However, R. Yochanan says (158a) that it is in the Chazakah of the husband's heirs, and "the property is in its Chazakah" is the reason for the law;

Why is this needed? We already know this, because one does not bequeath a Shevu'ah to his sons!

Answer #1: This is the very reason! Because the property is in the husband's Chazakah, she does not bequeath it;

If the property were in her Chazakah, she would bequeath it, like according to Beis Shamai, who say that it is as if it was collected.

Answer #2 (Ri): We can establish it to discuss intact Metaltelim. She collects them without a Shevu'ah.

Implied question: Why don't we give this answer in Shevu'os, rather, [we give] a weak answer, and establish it like Beis Shamai?

Answer: There, we discuss the Kesuvah. Here, we discuss Nichsei Tzon Barzel that she brought from her father's house. It is common that they are intact, for the husband does not normally sell them;

There is an opinion below (66b) that one who brought appraised property from her father's house, [even] if the husband wants to sell it, he may not sell.

Explanation: Even so, according to Beis Hillel they are in the Chazakah of the father's heirs;

Her Chazakah is stronger, for we should establish inheritance in the Chazakah of the Shevet [in which it used to be].

Implied question: We could have asked from several Mishnayos in Bava Basra (108a) which say that a husband inherits his wife!

Answer: We prefer to bring the Seifa of the Mishnah we brought above.

Explanation: This is because [Kinyan] ha'Guf (the property itself) belongs to her.

Implied question: Abaye himself holds like this in Kesuvos (81a. Why did he ask?!)

Answer: After he heard this from Rabah, he held like this.

Implied question: In Sotah (23b), a Mishnah says that an Arusah or Shomeres Yavam does not drink and does not receive a Kesuvah. Beis Shamai do not argue!

Answer #1 (Ri): There, she caused her own problem. She was secluded, and forbade herself to her husband;

Here, when she was secluded it was possible for her to be tested through drinking.

Answer #2: We consider an Arusah or Shomeres Yavam who was secluded to be Overes Al Das (acts unless the modest ways of Benos Yisrael. She forfeits her Kesuvah due to this.)

Implied question: In Sotah (24a), it says "they do not drink and do not receive a Kesuvah"!

Answer: There it is like the Gemara corrects the text here.

Question: Just the contrary! He should ask why they receive the entire Kesuvah due to Safek!

Answer: There, it is because she has a Vadai claim (I did not have Bi'ah), and the heirs have a Safek claim.