Reference for Yevamot 47:15

ואימא דמית קטן ואמר רחמנא לייבם בכור הא מיעט רחמנא אשת אחיו שלא היה בעולמו

since the All Merciful has made levirate marriage dependent on inheritance, and the right of inheritance derives from the father and not from the mother.<span class="x" onmousemove="('comment',' Hence there was no need to exclude him. The exclusion consequently indicates that by firstborn, in this context, any elder brother was meant. ');">³⁶</span> But might It not be suggested that where there is a firstborn the commandment of the levirate marriage shall be observed;<span class="x" onmousemove="('comment',' Either by the firstborn or by any other of the brothers, and that for this reason the exclusion of 'a brother who was not his contemporary' was necessary. ');">³⁷</span> where, however, there is no firstborn the commandment of the levirate marriage shall not be observed?<span class="x" onmousemove="('comment',' At all; by any brother. ');">³⁸</span>

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