Reference for Sanhedrin 151:1

מה להיא שכן אמה בשרפה תאמר בהוא שאמו בסקילה

[Thus]: Why is her case [i.e., his wife's maternal grandmother forbidden]? Because her mother is [forbidden] on pain of death by fire. But can you say the same in his case, seeing that his mother is forbidden on pain of stoning [only]? Further, his maternal grandmother is like her's: just as in the latter case no distinction is drawn between his wife's maternal grandmother and her [his wife's] daughter,<span class="x" onmousemove="('comment',' Incest with both being punishable by fire. ');">¹</span>

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