Reference for Yevamot 63:12

וסבר רבי יוסי איסור חל על איסור והא תניא עבר עבירה שיש בה שתי מיתות נידון בחמורה רבי יוסי אומר נידון בזיקה הראשונה הבאה עליו

here it is a case where the deceased had married first and the surviving brother afterwards.<span class="x" onmousemove="('comment',' Cf. previous two notes mutatis mutandis. ');">³⁵</span> As to R. Simeon, in the case where the deceased had married first and the surviving brother married afterwards, let her, since the prohibition of wife's sister cannot take effect, be permitted even to contract the levirate marriage! — R. Ashi replied: The prohibition of wife's sister remains suspended, and as soon as the prohibition of brother's wife is removed<span class="x" onmousemove="('comment',' [H], lit., 'to split', hence removed'. ');">³⁶</span> the prohibition of wife's sister comes into force; hence It cannot be treated as non-existent.<span class="x" onmousemove="('comment',' Lit., 'it is not removed'. The levirate marriage is consequently forbidden. ');">³⁷</span>

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