Reference for Yevamot 109:16

אחותו דכתב בה כרת למה לי

permitted after the death of her husband! — Scripture said, She is,<span class="x" onmousemove="('comment',' E.V. 'it is'. Lev. XVIII, 16, which speaks also, as deduced supra, of the wife of a maternal brother. ');">³⁵</span> she retains her status.<span class="x" onmousemove="('comment',' As she was forbidden to the levir during the lifetime of her husband she remains so after his death. ');">³⁶</span> What need was there to specify the penalty of <i>kareth</i> for intercourse with one's sister?<span class="x" onmousemove="('comment',' Her case, surely, is included in Lev. XVIII, 29, among all the others with whom intercourse is forbidden under the penalty of kareth! ');">³⁷</span>

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