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Text Excerpt
תינח היכא דנשא מת ואחר כך נשא חי. דמגו דאישתרי איסור אשת אח — אישתרי נמי איסור אחות אשה.
This works out well when the deceased brother had first married one of the sisters, who was thereby rendered prohibited to the yavam as his brother’s wife, and only afterward had the living brother married her sister, which renders his brother’s wife forbidden to him on another count, this time as his wife’s sister. As in that case one can say that since the first prohibition with regard to a brother’s wife was subsequently nullified and thereby permitted by the mitzva of levirate marriage, the prohibition with regard to a wife’s sister was also nullified and thereby permitted.
אלא נשא חי, ואחר כך נשא מת — איסור אחות אשה קדים.
However, if the reverse occurred, and first the living brother had married his wife and only afterward the deceased brother had married her sister, in this case the prohibition with regard to a wife’s sister precedes the prohibition proscribing a brother’s wife, i.e., she was initially forbidden to him as his wife’s sister. Consequently, even when the second prohibition lapses upon the death of his childless brother, the first prohibition should remain intact.
ואפילו נשא מת נמי, תינח היכא דנשא מת, ומת, ואחר כך נשא חי — דחזיא ליה דביני ביני. אלא נשא מת, ולא מת, ואחר כך נשא חי — לא איחזיא ליה כלל.
And furthermore, even in a case where the deceased brother had married first, it works out well only when the deceased brother had married the woman and then died, and only afterward the surviving brother married that brother’s wife’s sister. The reason is that in this case, the sister of the wife of the deceased brother was fit for him in between, i.e., during the period from when the deceased brother died until he married that brother’s wife’s sister. Therefore, it can be claimed that the second prohibition should not apply. However, if the deceased brother married first, and did not die yet, and afterward, during the deceased brother’s lifetime, the living brother married the deceased brother’s wife’s sister, she was not fit for him at all, as there was no period of time during which she was permitted to him.