Study Yevamot folio 95A with parallel Hebrew-English text, traditional commentary, and modern study tools. Free access to Babylonian Talmud online.
However, with regard to a wife’s sister, where even if the sister sins intentionally the wife is not forbidden to him by Torah law, if he did so unwittingly the rabbis did not decree with regard to him. And from where do we derive that she is not forbidden? As it is taught in a baraita that in the
As, were it not for this verse, one might have thought: Could this not be derived through an a fortiori inference: And if in a case where he has sex subject to a light prohibition, the one causing her to be rendered prohibited is forbidden, then in a situation where he has sex subject to a severe
R' Yehuda said: Beit Shammai and Beit Hillel did not disagree with regard to one who has sex with his mother-in-law, that he renders his wife disqualified from remaining married to him. With regard to what case did they disagree? With regard to one who has sex with his wife’s sister, as Beit Shammai
R' Yosei explains why Beit Shammai and Beit Hillel did not disagree with regard to the case of one who has sex with his wife’s sister. This is because at first, before the marriage, he is permitted to all the women in the world and she is permitted to all the men in the world. After he has betroth
R' Yosei’s explanation continues. Could this halakha of a wife’s sister not be derived through an a fortiori inference: And if he prohibited her through their betrothal to all men in the world, and yet she was unwitting with one forbidden to her, i.e., she had sex with another man by mistake, she