Quoting%20commentary for Yevamot 109:1

אלמא אחות אשתו בין מן האב בין מן האם אסורות מנלן יליף מאחותו מה אחותו בין מן האב בין מן האם אף כאן בין מן האב בין מן האם

From this it clearly follows that a wife's sister, whether she is paternal or maternal, is forbidden.<span class="x" onmousemove="('comment',' Since the third, the maternal sister of the second, is permitted only on account of the illegality of the marriage of the second, but is forbidden where the marriage with the second is legal. ');">¹</span> Whence, however, is this derived? — Deduction is made from one's sister; as a sister [is forbidden] whether she is paternal or maternal, so here also<span class="x" onmousemove="('comment',' A wife's sister is forbidden. ');">²</span> whether she is paternal or maternal. But let the deduction<span class="x" onmousemove="('comment',' In respect of a wife's sister. ');">³</span>

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