Reference for Keritot 28:34

דאיכא ברא לסבא מיגו דאיתוסף איסורא לגבי בריה איתוסף איסורא לגבי דידיה:

R'JOHANAN B. NURI REMARKED: IF ONE HAD INTERCOURSE WITH HIS MOTHER-IN-LAW HE MAY THEREBY BECOME GUILTY FOR OFFENDING WITH HIS MOTHER-IN-LAW, THE MOTHER OF HIS MOTHER-IN-LAW, AND THE MOTHER OF HIS FATHER-IN-LAW.<span class="x" onmousemove="('comment',' Viz., if in addition to the above inter-relations he had also been married to her daughter's daughter and her son's daughter, so that she was also his mother-in-law's mother and his father-in-law's mother.');">¹⁹</span>

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