Yevamot

with his wife's sister, however, presumptuous [marriage with whom does] not [cause his first wife to be] forbidden [to him] by Pentateuchal law, no preventive measure has been instituted by the Rabbis in her case where [he acted] unwittingly.  Whence, however, is it deduced that she  is not forbidden?  — [From that] which was taught: With her;  only cohabitation  with her causes her to be prohibited;  cohabitation  with her sister, however, does not cause her to be prohibited. [This, Scriptural text was required] since [otherwise] It might have been argued [as follows]: If where a man cohabited with [a woman forbidden by] a lighter prohibition.  [the person]  who caused the prohibition [itself]  is forbidden [to her],  how much more should [the person]  who caused the prohibition become forbidden in the case of cohabiting with [one  forbidden by] a heavier prohibition. R. Judah stated: Beth Shammai and Beth Hillel are agreed  that a man who cohabited with his mother-in-law renders his wife unfit [to live with him]; they only differ where a man cohabited with his wife's sister, in which case Beth Shammai maintain that thereby he causes [his wife] to be unfit for him, while Beth Hillel maintain that he does not thereby cause her to be unfit for him. R. Jose stated: Beth Shammai and Beth Hillel are agreed  that a man who cohabits with his wife's sister does not thereby render his wife unfit for him; they differ only where a man cohabited with his mother-in-law, in which case Beth Shammai maintain that thereby he causes [his wife] to be unfit for him, while Beth Hillel maintain that he does not thereby cause her to be unfit for him. [Both agree]  for the following reason:  Originally all the women of the world were permitted to him,  and all the men of the world were permitted to her;  but when he betrothed her he imposed a prohibition upon her and she imposed a prohibition upon him; the prohibition, however, which he imposed upon her is greater than the prohibition which she imposes upon him, since he caused all the men of the world to be forbidden to her, while she caused her relatives only to be forbidden to him. This,  then, may be arrived at by an inference: If she, to whom he  caused  all the men in the world to be prohibited, is, if she cohabited unwittingly with one who was forbidden to her,  not forbidden to the man  who was permitted to her,  how much more reason is there why he  to whom she  caused  the prohibition of her relatives only, should, if he cohabited unwittingly with one who was forbidden to him,  not be forbidden  to her  who was permitted to him. This argument is applicable to one who acted unwittingly. Whence is it deduced [that the same law  is applicable] to one who acted wilfully? It was expressly stated With her,  cohabitation  with her only causes her to be prohibited;  cohabitation  with her sister, however, does not cause her to be prohibited. Said R. Ammi in the name of Resh Lakish: What is R. Judah's reason?  — Because it is written, They shall be burnt with fire. both he and they;  is the whole household to be burned!  If this, then, is not a case for burning  regard the text as indicating a prohibition. Rab Judah stated in the name of Samuel: The law is not in agreement with R. Judah. A man once committed incest with his mother-in-law, and Rab Judah summoned him and ordered him to receive a flogging. 'Had Samuel not stated', he said to him, 'that the law was not in agreement with R. Judah. I would have forbidden [your wife] to you for all time'. What was meant by a 'lighter prohibition'?  — R. Hisda replied: Remarrying one's divorced wife after her marriage to another man — 39 When that man  cohabited with her, he caused her to be prohibited to the other,  and when the other  cohabited with her  he caused her to be prohibited to the former.  [But, it may be argued,] remarrying one's divorced wife after her marriage to another man is different  since her body  was defiled and she is  prohibited for all time!  — Rather, said Resh Lakish, [it means] a yebamah. A yebamah with whom?  If it be suggested: With a stranger,  [the ruling] being in accordance with R. Hamnuna who ruled  that a woman awaiting the decision of the levir who played the harlot is forbidden to the levir,  [it may be objected that] a yebamah is different,  since her body was defiled and she is prohibited to the majority of men.  If, however, [it be suggested that it refers  to] a yebamah in relation to [her deceased husband's] brothers: Where one [brother, for instance] addressed to her a ma'amar he caused her to be prohibited to the other,  and when the other cohabited with her he caused her to be prohibited to the former.  [But in this case] what point is there, [it may be retorted, in stating]  that the second cohabited with her,  [when the same law is applicable] also even where he  only addressed to her a ma'amar!  — This is no difficulty; [a ma'amar could not be postulated], in accordance with R. Gamaliel who ruled: There is no validity in a ma'amar that was addressed after a previous ma'amar.  But [still the objection is that the same law is applicable] even if he  gave her a letter of divorce and even if he submitted to her halizah! — Rather, said R. Johanan, [it means] a sotah. A sotah, with whom?  If it be suggested: With her husband who, if he cohabited with her,  caused her to be prohibited to her seducer,  what point is there, [it may be objected, in stating] that he cohabited with her? Even if he  only gave her a letter of divorce and even if he only said, 'I am not allowing her to drink',  [the same law is applicable]!  [If it be suggested] however: The sotah with the seducer;  is this  [it may be objected] a 'lighter prohibition'? It is surely a grave prohibition, since she is a married woman!