Yevamot 22A

הנך תרתי דדמיין להדדי כחדא חשיב להו, והא שיתסרי. והא מכל מקום לדידי חזיין לן דכתיבן לאיסורא! אמר ליה: וליטעמיך, אי הוה כתיב להיתירא, מי הוות סמכת עלייהו? דמר בריה דרבנא מי חתים עלייהו? השתא נמי דכתיב לאיסורא ⁠— לאו מר בריה דרבנא חתים עלייהו.

Those two, the wife of a father’s father’s brother and the sister of a father’s father, are similar to each other and are counted as a single case, and so there are 16. The Talmud restates Rav Hillel’s challenge to Ameimar’s opinion: But in any case I myself saw them written as prohibited. Rav Ashi said to Rav Hillel: And according to your reasoning, if in the list it was written that they were permitted, would you have relied on that? Is the signature of Mar, son of Rabbana, signed on it? Although you saw the list, you don’t know for certain who wrote it. Now too, that it is written that they are prohibited, the same holds true; the signature of Mar, son of Rabbana, is not signed on it, and so it cannot be relied upon to reject the opinion of Ameimar.

תני דבי רבי חייא: שלישי שבבנו ושבבתו ושבבן אשתו ושבבת אשתו ⁠— שניה. רביעי שבחמיו ושבחמותו ⁠— שניה.

§ One of the rabbis of the school of R' Ḥiyya taught: The third generation from one’s son and one’s daughter, i.e., one’s great-grandchildren, and the third generation from his wife’s son and his wife’s daughter, i.e., one’s wife’s great-grandchildren, are all forbidden as secondary forbidden relationships. So too, the 4th generation from his father-in-law and from his mother-in-law, i.e., his wife’s great-grandmothers, are prohibited as secondary forbidden relationships.

אמר ליה רבינא לרב אשי: מאי שנא למעלה דקחשיב לה לאשתו, ומאי שנא למטה דלא קחשיב לה לאשתו? למעלה, דאיסורא מכח אשתו קא אתי ⁠— חשיב לה, למטה, דאיסורא לאו מכח אשתו קאתי ⁠— לא קחשיב לה.

Ravina said to Rav Ashi: What is different between the generations above, referring to his wife’s great-grandmothers, such that the Sage counts his wife as one of the generations and refers to them as the 4th generation, and what is different with regard to the generations below, referring to his great-granddaughters, such that he did not count his wife and referred to them as only the third generation? The Talmud answers: When counting above, the prohibition stems from his wife, as they are not his blood relatives but his wife’s. Therefore, she is counted. When counting below, however, the prohibition does not stem from his wife, as they are his own blood relatives, and so she was not counted.