Question: If tht is so, when the Mishnah lists 'brother-in-law, son and son-in-law', the latter means 'the son-in-law of one's son', then it will emerge that the 'son-in-law of the son of his wife's sister' is Pasul. But how can that be, when, based on the statement 'stepson alone', even the son-in-law of his wife's son is permitted?

Answer: Since 'son-in-law' now means 'the son-in-law of one's son', by the same token 'stepson alone' does not come to preclude 'the stepson's son and son-in-law', but rather 'the son and the son-in-law of his stepson's son'.

Implied Question: Many of them are really one and the same ...

Answer: But the Gemara is merely following the way the Mishnah lists them.

Implied Question: Why does R. Chiya not then say 'forty' - they, their sons, their sons-in-law, the sons-in-law of their sons and the sons of their sons?

Answer #1: Because they (' the sons-in-law of their sons') are not mentioned in the Mishnah.

Answer #2: Alternatively, because the term 'Chasnus' is one and the same.

Question: Rashi asks why the Gemara does not query Rav in that now that we have said they are 'like another generation', why is 'son-in-law' Pasul to his father's brother, seeing as he is a 'fourth to a first'?

Answer: Because it asks a different Kashya (whose answer answers that Kashya too).

Question: ha'Rav Rav Yosef asked his brother Rabeinu Peretz why we do not query Rav, who declares Kasher 'a third to a second', from the statement 'My father's brother, his son and his son-in-law, cannot testify against me', in which case 'a third is Pasul to a second'?

Answer: Rabenu Peretz replied that the Gemara could indeed have asked that, only it asked a different Kashya (just like Rashi replied to the previous question).

Ruling: Rabeinu Tam writes that this opinion is Halachah - that 'the third to the first' is Pasul, as is evident in Perek Yesh Nochlin (Bava Basr, 58b & 150a) where R. Aba sent to Rav Yosef bar Chama that 'the third to the second is Kasher', implying to the first, he is Pasul, and this is the Halachah, as the Gemara rules there.

Support: The Yerushalmi also conforms to this ruling, when it asks whether Moshe was permitted to testify for/against Pinchas' wife, to which R. Yochanan replies in the affirmative, implying that he may not testify for/against Pinchas himself - because that would be a third to a first.

Implied Question: And the reason that in this case, we do not apply the principle that 'a woman is considered like her husband' (like we say by 'a stepson's wife') is ...

Answer: ... because there it is one generation removed (so the Chachamim were more lenient).

Clarification: They are not exactly similar, since 'son-in-law alone' comes to preclude both 'son and son-in-law'; whereas 'brother-in-law alone' does notvpreclude 'his son' - since the author of the Beraisa is R. Yossi - So we see that it reckons 'the husband of his mother's sister, which equivalent to his brother-in-law's son, and it only comes to preclude his 'son-in-law' as Rashi explains.

Halachic Inference: Since the Gemara concludes that the Halachah is like R. Yossi (that 'brother-in-law only' comes to preclude 'son-in-law'), we can extrapolate that if two people marry the daughters of two sisters or of two brothers, they are certainly Kasher.

Inference (Part 1): Abaye's answer indicates that he follows the opinion of R. Elazar (that it is the witnesses who witness the handing over of the Sh'tar who determine ownership.

Inference (Part 2): Consequently, Abaye will hold that, if one documents something in the name of Reuven and then hands it to Shimon, it is Shimon who will acquire it.

Question: But how can he say that, when in the first Perek of Bava MeTzi'a, (Daf 13a & 13b) he rules that witnesses with their signatures acquire on behalf of the recipient?

Answer (Part 1): The case under discussion, where 'he wrote it for one person' speaks where the two claimants produce two Sh'taros that are dated on the same day, and one cannot tell which was written first. That is where Abaye goes after the Mesirah (since his principle is not applicable).

Answer (Part 2): But where it is evident from the Sh'tar that the owner received it first, then Abaye abides by his principle, and he gets it, even if the owner subsequently gave it to somebody else.