For the source text click/tap here: Yevamot 3
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Even though the commandment of yibum (levirate marriage) overrides the prohibition of a man marrying his sister-in-law, it would not permit him to marry her if she is an immediate relative who he cannot marry (e.g. his daughter, niece or mother-in-law).
In searching for a source for this halakha, the Gemara quotes a baraita that highlights the word aleha – “upon her” – which appears in both the list of forbidden relatives (Vayikra 18:18) and the commandment of yibum (Devarim 25:5). The Torah‘s use of the same word in both places is understood to teach that even in the presence of a potential mitzvah, the prohibition remains in place.
We continue our exploration of levirate marriage in antiquity and the curious case in Charleston SC 1807 as well as the famous Yeshiva of the Node Bi’Yehudah.