Reference for Yevamot 188:13

ותאסר בשכיבה דאחותה מידי דהוה אאשה שהלך בעלה למ"ה לא דמי אשתו דבמזיד אסירא מדאוריי' בשוגג גזרו בה רבנן

This presents no difficulty according to Abaye who explained that the difference between them<span class="x" onmousemove="('comment',' R. Ishmael and R. Akiba. ');">⁴³</span> lies in the interpretation of the text,<span class="x" onmousemove="('comment',' Lev. XX. 14, speaking of a man who take with his wife also her mother (ibid.). ');">⁴⁰</span> R. Ishmael maintaining that the text mentioned only one<span class="x" onmousemove="('comment',' Forbidden woman (v. supra n. 10). the first having been lawfully married. ');">⁴⁴</span> while R. Akiba maintains that the text spoke of two.<span class="x" onmousemove="('comment',' Women that were both forbidden to the man; where, for instance, he married his mother-in-law and her mother. According to this explanation of Abaye the question of marrying a mother-inlaw after the death of one's lawful wife did not arise in the dispute, and R. Akiba's opinion on the subject cannot, therefore, be inferred from it. ');">⁴⁵</span> According to Raba, however, who explained that the difference between them<span class="x" onmousemove="('comment',' R. Ishmael and R. Akiba. ');">⁴³</span> is [the case of marriage of] a man's mother-in-law after the death [of his wife].<span class="x" onmousemove="('comment',' R. Ishmael maintaining that even when a man had married his mother-in-law after the death of his wife he is to be burned, while R. Akiba maintains that he is burned only if both women were alive. (Cf. Sanh. 76b). ');">⁴⁶</span> his mother-in-law should also have been mentioned!<span class="x" onmousemove="('comment',' In our Mishnah; since, as has been shewn, according to Raba's explanation, marriage of a mother-in-law after the death of her daughter is, according to R. Akiba, permitted ');">⁴⁷</span> — The other replied: Granted that Scripture has excluded her<span class="x" onmousemove="('comment',' A mother-in-law that was married by her son-in- law. ');">⁴⁸</span> from the penalty of burning. has Scripture. however, excluded her from the prohibition?<span class="x" onmousemove="('comment',' Evidently not. Her case, therefore, could not have been mentioned in our Mishnah. ');">⁴⁹</span> Let her,<span class="x" onmousemove="('comment',' The first wife spoken of in our Mishnah, who IS PERMITTED TO RETURN TO HIM. ');">⁵⁰</span> however, be forbidden [to her husband] through his cohabitation with her sister, her case being similar to that of a woman whose husband went to a country beyond the sea!<span class="x" onmousemove="('comment',' And she married a second husband. In both cases the women acted unwittingly. As in the latter case the woman is forbidden to her husband, so should the woman in the case in our Mishnah. ');">⁵¹</span> — [The two cases are] not alike: His wife who, [if she had acted] presumptuously,<span class="x" onmousemove="('comment',' In marrying a second husband. ');">⁵²</span> is forbidden to him by Pentateuchal law, has been forbidden to him, when [she acted] unwittingly, by a preventive measure of the Rabbis;

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