Reference for Yevamot 47:19

ואימא כי מייבם בכור לישקול נחלה כי מייבם פשוט לא לישקול נחלה אמר קרא יקום על שם אחיו והרי קם

May it be suggested that where there is a firstborn one turns to the eldest<span class="x" onmousemove="('comment',' If the other brothers refused to marry the widow it should be his duty to marry her. ');">⁴⁷</span> but where there is no firstborn one does not turn to the eldest?<span class="x" onmousemove="('comment',' Not being the firstborn it is no more his duty to marry the widow than it is that of his brothers. ');">⁴⁸</span> Why, then, did Abaye the Elder teach that the commandment to perform the duty of the levirate marriage is incumbent Upon the elder brother; if he refuses, the younger brother is approached;<span class="x" onmousemove="('comment',' I.e., all the brothers are approached in the order of seniority. V. Tosaf. s.v. [H], a.l., and cf. Rashi a.l. ');">⁴⁹</span>

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