Answer #1: The Ri answers that if there is a lot of hair, we rely on that, since it is not common for this to happen before thirteen years.

Answer #2: Also, if they are tall in stature, one can rely on the Simanim and the size, even if he only has two pubic hairs ...

Conclusion: ... whereas our Sugya is speaking where there are only two hairs and he is not tall; hence the Gemara says that he (the father) is not believed regarding Makos and punishments.

Explanation #2: Furthermore the Ri explained differently than Rashi - that where there are Simanim testimony of age is not required.

Question: In that case, why do we need the father's testimony?

Answer: Because it speaks here where the father testifies that a year or two have already elapsed since his son ate Cheilev or committed incest, and that he was a Gadol when he did ...

Answer (cont.): ... only we don't know if he was of age or he had the Simanim at that time; and the Gemara teaches us that the father is believed to say that he was a Gadol or that he had Simanim - two pubic hairs.

Implied Question: ... and the reason that Rav Chisda did not mention Simanim is ...

Answer: ... because he mentioned one of two possibilities.

Reason: Because one witness is believed with regard to Isurin.

Question: He is only believed if it is be'Yado (in his hands to achieve it [See Masores ha'Shas])?

Answer: Since he stands to become a Gadol, they considered it a Davar she'be'Yado.

Question: In that case, why specifically his father? Why not anybody else?

Answer: Because a father is more particular to take note of his age ...

Answer (cont.): ... since the onus lies on him to inform others; Hence it is specifically concerning him that they considered it as be'Yado, but not concerning anybody else.

Explanation #1: Seeing as his daughter is Kasher, Rashi explains, so is his widow.

Explanation #2: The Ri however, explains it differently. According to him, if he would render his widow Pasul with his Bi'ah, the children would not be Kasher

Reason: ... since it would then be like a Chalal who marries a Chalalah, whose child is a Chalal, according to all opinios (later, on Daf 74b). Tosfos Yeshanim].

Explanation #1 (Part #1): Rashi explains that the time of Kidushin is considered like witnesses and negates the Chazakah, since we initially assumed that she had brothers but no sons ...

Explanation #1 (Part #1 [cont]): In which case the woman in question has a Chazakah of being not subject to Yibum ...

Explanation #1 (Part #1 [cont]): ... and when he now comes at the time of death to take her out of her Chazakah, he is not empowered to do so.

Part #2: And Rebbi Nasan holds that 'he could have told a bigger lie' (Migu) is on a par with Chazakah and a Chazakah cannot negate a Chazakah ...

Clarification: Initially this woman had a Chazakah that she was subject to Yibum, seeing we knew that he (her Chasan) had brothers but no sons ...

Clarification (cont.): So when he then claims at the time of the Kidushin that he had sons, had he not retracted at the time of death, we would have believed him due to the Migu ...

Clarification (concl.): ... inasmuch as had he wanted to exempt her from Yibum, he could have given her a Get a short time prior to his death ...

Refutation: [The Ri does not agree however, that he should be believed via what he said at the time of Kidushin, when he intended to retain her?

Explanation #2: He therefore explains that the Migu is that if he wanted, he could have written a Get a the time of Kidushin to divorce her whenever he wanted ...

Implied Question: ... despite the fact that it is forbidden to divorce with an old Get ...

Answer: ... it is considered a Migu, seeing as Bedi'eved it is valid (Gitin, 79b). Tosfos Yeshanim].

Explanation #2 (cont.): But when he retracts at the time of death, we establish the initial Chazakah ...

Reason: ... because he holds that a Migu is like a Chazakah, which cannot negate the Chazakah that existed prior to the Kidushin.

Explanation #2 (cont.): However, if he would not retract, she would be permitted to get married - since 'A person does not sin on somebody else's account' (Above, Daf 63b) ...

Explanation #2 (concl.): ... and if she was really subject to Yibum, he would have retracted at the time of death.

Question: In that case, why do we establish the Mishnah where he was not Muchzak to have brothers? Why is he not believed even if he did, when he says that he has sons ...

Reason: ... for the reason that Rashi gives - that 'A person does not sin on somebody else's account'?

Answer #1: We have to establish the Mishnah like that, because it states 'If he has brothers, he is not believed'.

Answer #2: Alternatively, because if he was Muchzak to have brothers, he would not be believed to say at the time of death that he has sons ...

Reason: ... because we cannot say at the time of death that he could have released her with a Get, since he may not have had the strength to do so.