Yevamot

is an instruction as to what it is the proper thing to do.  Let him reply that it  was a preventive measure against the possibility of the levir's participating first in the halizah of the first!  — It was stated, BUT MAY NOT BE TAKEN IN LEVIRATE MARRIAGE, i.e., the law of the levirate marriage is not applicable here at all.  Let him, then, reply that it  was a preventive measure in case he  might die,  it being forbidden to annul the precept of levirate marriage!  — R. Johanan makes no provision against possible death.  Then let him reply that it  is the ruling of R. Eleazar  who said that so long as she remained forbidden to him for one moment she is forbidden to him for ever!  — Since the latter clause [represents the view of] R. Eleazar,  the first clause cannot represent his view. Then let him reply that it  is a case where they  fell under the obligation  at the same time, and that it represents the opinion of R. Jose the Galilean who maintains that it is possible to ascertain simultaneity!  — The Tanna would not have recorded an anonymous Mishnah in agreement with the view of R. Jose the Galilean. Let him reply [that it  is a case] where it is not known which  came under the obligation  first!  — If that were the case  how could it have been stated,  EVEN IF THEY HAD ALREADY MARRIED THEM THEY MUST DISMISS THEM! In the case of the first,  at least, one can understand [the reason].  since he can be told, 'Who permitted her to you'?  In the case, however, of the second,  the levir  could surely claim, 'My friend  has taken the second in levirate marriage  and I take the first '  This, then,  is the reason why he  said to him,  'I do not know who was the author of the statement concerning the sisters'. We learned: IF ONE OF THE SISTERS WAS FORBIDDEN TO ONE [OF THE BROTHERS] UNDER THE PROHIBITION OF INCEST,  HE IS FORBIDDEN TO MARRY HER BUT MAY MARRY HER SISTER, WHILE TO THE SECOND BROTHER BOTH ARE FORBIDDEN. It was now assumed that his mother-in-law  came under the obligation  first.  Now, why [should both sisters be forbidden]?  Let the son-in-law undertake the duty of marrying first that sister who is not his mother-in-law,  and his mother-in-law, in relation to the other levir, would thereby come into the same category as a sister-in-law that was permitted,  then forbidden,  and then permitted again,  who returns to her former state of permissibility! R. Papa replied: [They are forbidden] in a case where she who was not his mother-in-law came under the obligation  first. R. ELIEZER SAID: BETH SHAMMAI HOLD etc. The following was taught: R. Eliezer said: Beth Shammai hold that they may retain them, and Beth Hillel hold that they must dismiss them. R. Simeon said: They may retain them. Abba Saul said: Beth Hillel uphold in this matter the milder rule, for it was Beth Shammai who said that the women must be dismissed while Beth Hillel said they may be retained. Whose view does R. Simeon represent?  If that of Beth Shammai,  he is merely repeating R. Eliezer; if that of Beth Hillel,  he is repeating Abba Saul! It was this that he meant: In this matter there is no dispute at all between Beth Shammai and Beth Hillel. IF ONE OF THE SISTERS etc. But we have learned this already: When her sister is her sister-in-law she may either perform halizah or be taken in levirate marriage!  — [Both are] necessary. For had the law been stated there  it might have been assumed [to apply to that case alone],  because there is no need to enact a preventive measure against a second brother,  but not [to the case] here where it might be advisable to issue a preventive measure against a second brother.  And had the law been stated here,  it might have been assumed [to apply to this case alone] because there is a second brother who proves it  but not [to that case] where no second brother exists.  [Hence were both] required. BY VIRTUE OF A COMMANDMENT etc. But we have [already] learned this also: