Reference for Sanhedrin 149:14

לרבא איידי דקא בעי למיתנא אם חמיו ואם חמותו תני נמי חמותו

since the Mishnah desires to state — HIS FATHER-IN-LAW'S MOTHER, it adds HIS MOTHER-IN-LAW AND HER MOTHER. On Raba's view,<span class="x" onmousemove="('comment',' That only his mother-in-law is explicitly forbidden on pain of death by fire, but not her mother. ');">¹¹</span>

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