Yevamos, Chapter 10, Mishnah 4. This mishnah deals with cases in which a wife traveled overseas and a report arrived that she had died, and on the basis of that report her husband married her sister - since after a wife's death her sister is permitted to him.
The first case:
"Mi shehalchah ishto limdinas hayam, uva'u ve'amru lo: meisah ishtecha, venasa es achosah, ve'achar kach ba'as ishto - muteres lachzor lo" - his wife traveled overseas, a report of her death reached him, and on its basis he married her sister. When his wife returned and it turned out that the report was false, he is permitted to return to her, and the fact that he had forbidden relations with her sister does not render his wife forbidden to him.
The Gemara derives this from the verse stated regarding a sotah: "Veshachav ish osah" - his lying with her renders her forbidden, but her sister's lying does not render her forbidden. In other words, the husband's forbidden relations with her sister do not render the wife forbidden, and therefore he may return to her.
The mishnah continues and spells out the remaining ramifications:
"Hu mutar bikrovos sheniyah" - the husband is permitted to the relatives of the second woman, the sister whom he attempted to marry, since the kiddushin never took effect at all.
"Vesheni mutar bikrovav" - and likewise the sister is permitted to his relatives, since there was never any kiddushin between them at all.
"Ve'im meisah rishonah - mutar basheniyah" - if his first wife later actually died, he is permitted to go back and marry the sister, since what happened has no effect whatsoever.
The second case:
"Amru lo: meisah ishtecha, venasa es achosah, ve'achar kach amru lo: kayemes haysah umeisah - havlad harishon mamzer, veha'acharon eino mamzer" - they told him that his wife had died and he married her sister, and afterward it became clear to him that at the time he married the sister his wife was still alive, and only later did she die. It emerges that at the time of the marriage the sister was forbidden to him, and therefore the first child - born or conceived while the wife was still alive - is a mamzer; whereas the child born after the wife actually died is not a mamzer.
From here the mishnah moves on to the statement of Rabbi Yosi, which is one of the most complex cases in Maseches Yevamos. To understand it, we need one introduction:
Until now, and especially in the first mishnah of our chapter, we learned that a woman who heard that her husband had died overseas and married another man, and afterward it turned out that her husband was alive and returned, requires a get from her first husband. We understood that this is a penalty, based on the expectation that she should have been extremely careful and investigated the matter thoroughly, and since she did not do so she must bear the consequences and receive a get from her husband.
The case Rabbi Yosi discusses:
Two men who are not related to one another, Reuven and Moshe, are married to two sisters - and thus became brothers-in-law through their marriages:
Reuven is married to Leah.
Moshe is married to Rachel, Leah's sister.
Leah, Reuven's wife, traveled overseas with her brother-in-law Moshe, while Reuven and Rachel remained behind. A report arrived that both of them, Moshe and Leah, had died, and on that basis Reuven married Rachel - who is both his wife's sister and Moshe's wife, and since both of them are no longer alive she is permitted to him. In the end, Leah and Moshe appear and return.
The view of the Tanna Kamma:
Rabbi Yosi understood that according to the Tanna Kamma, although it is not stated explicitly, there is no difference between a case where Leah was Reuven's full wife through actual marriage and a case where she was only betrothed to him through kiddushin: in either case Rachel cannot return to Moshe when he comes back, as we learned in the first mishnah, since she married Reuven unlawfully; whereas Leah is permitted to return to Reuven, as we learned at the beginning of our mishnah, that a woman is not rendered forbidden to her husband because he married her sister.
The view of Rabbi Yosi:
Rabbi Yosi distinguishes between a case where an actual marriage took place between Reuven and Leah and a case where there was only kiddushin between them:
There was only kiddushin between them: There is room to be concerned that people will think that there was a condition attached to the kiddushin of Reuven and Leah, and that the condition was not fulfilled, so that they were in fact never married at all. Consequently, onlookers may believe that Reuven married her sister Rachel lawfully, and that Moshe divorced his wife Rachel - and even though Rachel is Leah's sister, that marriage never took effect because the condition was not fulfilled, and therefore he was permitted to marry her. And since people may regard the marriage of Reuven and Rachel as legitimate, Reuven must give Rachel a get. And once he has given her a get, she is no longer permitted to return to Moshe, since it would appear as though he is taking back his divorcee after she had married another man.
There was an actual marriage between them: We are not concerned that there was a condition, since we do not believe that a person enters into marriage conditionally - for by doing so he would turn his marital relations into licentiousness and forbidden relations, and a person does not do that. Therefore Reuven need not give Rachel a get, and Rachel is permitted to return to her husband Moshe. And since no get was given from Reuven to Rachel, Leah too is permitted to return to her husband Reuven.
It emerges that just as Reuven disqualifies Rachel from Moshe by means of the get he gave her, so too he ends up disqualifying Leah as well, with whom he had kiddushin, from himself - because she is the sister of his divorcee.
This is what Rabbi Yosi says in the mishnah:
"Kol shepose al yedei acheirim - posel al yedei atzmo" - when Reuven gives Rachel a get and thereby disqualifies her from Moshe, he also disqualifies his own Leah from himself.
"Vechol she'eino posel al yedei acheirim - eino posel al yedei atzmo" - in the scenario where an actual marriage took place, and Reuven does not give Rachel a get and does not disqualify her from her husband Moshe, he likewise does not disqualify his wife Leah from himself, since once no get was given to Rachel, Leah is not the sister of his divorcee.
In summary: In this mishnah we learned that one who marries his wife's sister on the basis of a mistaken report that she had died is permitted to return to his wife, as it says "Veshachav ish osah", his lying with her renders her forbidden but her sister's lying does not render her forbidden, and he is even permitted to the relatives of the second woman and likewise the reverse; that if it turned out that his wife was alive at the time of the marriage, the first child is a mamzer and the later one is not a mamzer; and finally we examined the dispute between the Tanna Kamma and Rabbi Yosi in the case of two brothers-in-law married to two sisters, and Rabbi Yosi's distinction between kiddushin and marriage, from which the rule "whoever disqualifies through others disqualifies through himself" is derived.