Sanhedrin 76A

מה להיא ⁠– שכן אמה בשרפה, תאמר בהוא ⁠– שאמו בסקילה?

What is notable about her relatives? They are notable in that one who has sex with the mother of his wife is liable to be executed by the more severe punishment of burning. Will you then say that there is a prohibition with regard to his relatives, where one who has sex with his mother is liable to be executed by the less severe punishment of stoning?

ועוד, הוא כהיא: מה היא ⁠– לא חלקתה בה בין בתה לאם אמה, אף הוא ⁠– לא תחלוק בו בין בתו לאם אמו.

And furthermore, one must say that the status of his relatives is like the status of her relatives; and just as with regard to her relatives, you did not distinguish between her daughter and the mother of her mother, and one who has sex with either is executed by burning, so too with regard to his relatives, you should not distinguish between his daughter and the mother of his mother and he should receive the same punishment for having sex with either. Since one who has sex with the mother of his mother is executed by stoning and one who has sex with his daughter is executed by burning, the parallel between his relatives and her relatives is incomplete, and the prohibition cannot be derived.

ולמאן דאמר סקילה חמורה, מהאי קושיא לא נידונה.

The Talmud continues: And according to the one who says that stoning is more severe than burning, due to this third difficulty, the fact that sex with the mother of his mother is forbidden, this halakha is not derived, as the parallel between his relatives and her relatives is incomplete.

אי: מה הוא ⁠– כלתו אסורה, אף היא ⁠– כלתה אסורה?

The Talmud asks: If a parallel is drawn between his relatives and the relatives of his wife with regard to the prohibition, just as with regard to his relatives, his daughter-in-law is forbidden to him, so too with regard to her relatives, her daughter-in-law is prohibited to him?