Yevamot

MISHNAH.IF TWO OF THREE BROTHERS WERE MARRIED TO TWO SISTERS AND THE THIRD WAS MARRIED TO A STRANGER, AND ONE OF THE SISTERS' HUSBANDS DIED AND THE BROTHER WHO WAS MARRIED TO THE STRANGER MARRIED HIS WIFE AND THEN DIED HIMSELF, THE FIRST  IS EXEMPT  AS BEING A WIFE'S SISTER, AND THE SECOND IS EXEMPT  AS BEING HER RIVAL. IF, HOWEVER, HE HAD ONLY ADDRESSED TO HER  A MA'AMAR AND DIED, THE STRANGER IS TO PERFORM THE HALIZAH BUT MAY NOT CONTRACT THE LEVIRATE MARRIAGE. GEMARA. The reason  is because he had addressed to her  a ma'amar;  had he, however, not addressed a ma'amar to her,  the stranger also would have had to be taken in levirate marriage.  This proves, said R. Nahman, that no levirate bond exists  even in the case of one brother. MISHNAH. IF TWO OF THREE BROTHERS WERE MARRIED TO TWO SISTERS AND THE THIRD WAS MARRIED TO A STRANGER, AND WHEN THE BROTHER WHO WAS MARRIED TO THE STRANGER DIED, ONE OF THE SISTERS' HUSBANDS MARRIED HIS WIFE AND THEN DIED HIMSELF, THE FIRST  IS EXEMPT  IN THAT SHE IS HIS WIFE'S SISTER, AND THE OTHER  IS EXEMPT AS HER RIVAL. IF, HOWEVER, HE HAD ONLY ADDRESSED TO HER  A MA'AMAR AND DIED, THE STRANGER MUST PERFORM HALIZAH  BUT MAY NOT BE TAKEN IN LEVIRATE MARRIAGE. GEMARA. What need was there again [for the law in this Mishnah]? Surely it is the same:  If there,  where the wife's sister is only a rival to the stranger  it has been said that the stranger is forbidden,  how much more so  here where the stranger is the rival to a wife's sister!  -The Tanna had taught first this,  while the other  was regarded by him as a permissible case, and so he permitted her.  Later, however, he came to regard it as a case that was to be forbidden;  and, as it was dear to him,  he placed it first; while the other Mishnah  was allowed to stand in its original form. MISHNAH. IF TWO OF THREE BROTHERS WERE MARRIED TO TWO SISTERS AND THE THIRD WAS MARRIED TO A STRANGER, AND WHEN ONE OF THE SISTERS' HUSBANDS DIED THE BROTHER WHO WAS MARRIED TO THE STRANGER MARRIED HIS WIFE, AND THEN THE WIFE OF THE SECOND BROTHER DIED, AND AFTERWARDS THE BROTHER WHO WAS MARRIED TO THE STRANGER DIED ALSO, BEHOLD, SHE  IS FORBIDDEN TO HIM  FOR ALL TIME, SINCE SHE WAS FORBIDDEN TO HIM FOR ONE MOMENT. GEMARA. Rab Judah said in the name of Rab: Any yebamah to whom the instruction Her husband's brother shall go in unto her  cannot be applied at the time she becomes subject to the levirate marriage, is indeed like the wife of a brother who has children, and is consequently forbidden.  What new thing does he  teach us? Surely we have learned, SHE IS FORBIDDEN TO HIM FOR ALL TIME SINCE SHE WAS FORBIDDEN TO HIM FOR ONE MOMENT! — It might have been assumed that this  applies only to the case where she  was not suitable for him  at all during the period of her first subjection;  but that where she  was at all suitable for him  during her first subjection  it might have been assumed that she  should be permitted, hence, he  taught us [that It was not so]. But we have learned this also: If two brothers were married to two sisters, and one of the brothers died and afterwards the wife of the second brother died, behold, she  is forbidden to him for all time, since she was forbidden to him for one moment!  — It might have been assumed [that this law is applicable] only there because she was completely forced out of that house;  but here, where she was not entirely forced out of that house,  it might have been said that as she is suitable for the brother who married the stranger she is also  suitable for the other brother,  hence he  taught us [that she was not]. MISHNAH. IF TWO OF THREE BROTHERS WERE MARRIED TO TWO SISTERS AND THE THIRD WAS MARRIED TO A STRANGER, AND ONE OF THE SISTERS' HUSBANDS DIVORCED HIS WIFE, AND WHEN THE BROTHER WHO WAS MARRIED TO THE STRANGER DIED HE WHO HAD DIVORCED HIS WIFE MARRIED HER AND THEN DIED HIMSELF- THIS IS A CASE CONCERNING WHICH IT WAS SAID: AND IF ANY OF-THESE DIED OR WERE DIVORCED. THEIR RIVALS ARE PERMITTED. GEMARA. The reason  is because he  had divorced [his wife first] and [his brother]  died afterwards,  but [if the other]  had died [first] and he  divorced [his wife] afterwards,  she  is forbidden.  Said R. Ashi: This proves that a levirate bond exists,  even where two brothers are involved. But as to R. Ashi's [inference] does not that of R. Nahman  present a difficulty? — R. Ashi can answer you: The same law, that the stranger is to perform the halizah and that she is not to be taken in levirate marriage.is applicable  even to the case where no ma' amar had been addressed; and the only reason why ma'amar was at all mentioned  was in order to exclude the ruling of Beth Shammai. Since they maintain that a ma'amar constitutes