Reference for Yevamot 63:11

אמר רב אשי איסור אחות אשה מיתלא תלי וקאי אי פקע איסור אשת אח אתי איסור אחות אשה וחייל והילכך לא פקע

and 'his wife's sister';<span class="x" onmousemove="('comment',' So that if the offence was committed unwittingly he is liable to bring two sin.offerings. ');">³²</span> so R. Jose. R. Simeon said: He is guilty on account of 'his brother's wife' on]y. But, surely. it was taught that R. Simeon said: He is guilty on account of 'his wife's sister' only! — This is no difficulty: There, it is a case where the surviving brother had married first<span class="x" onmousemove="('comment',' One of the sisters; and thus the prohibition of 'wife's sister' came into force first. ');">³³</span> and the deceased had married afterwards;<span class="x" onmousemove="('comment',' The other sister. The added prohibition of 'brother's wife' could not take effect where one prohibition was already in force. ');">³⁴</span>

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