Keritot 14B

גמ׳ והא לית ליה לרבי מאיר איסור חל על איסור! נהי דאיסור חל על איסור לית ליה, איסור מוסיף ואיסור כולל אית ליה.

Talmud: The Talmud asks: How could one become liable to bring multiple sin offerings? But R' Meir does not hold that a prohibition takes effect where another prohibition already exists. The Talmud answers: Although he generally does not hold that a prohibition takes effect where another prohibition already exists, with regard to an expanded prohibition and a more inclusive prohibition, he does hold that it takes effect where another prohibition already exists.

כגון שבא על אמו והוליד בת, דאיסור בתו ואחותו בהדי הדדי אתי. נשאת לאחיו, מיגו דאיתוסף איסור לגבי אחין ⁠– איתוסף איסורא לגבי דידיה.

Consequently, the ruling stated in the Mishnah applies in a case where one had sex with his mother and thereby fathered a daughter, so that the prohibitions against having sex with his daughter and with his sister come into effect simultaneously. If that daughter married his brother, then since a prohibition was added with regard to his other brothers, to whom she has now become forbidden, it is an expanded prohibition, and therefore a further prohibition is added with regard to him as well.

נשאת לאחי אביו, מיגו דאיתוסף איסורא לגבי שאר אחי אביו ⁠– איתוסף לגבי דידיה. הויא לה אשת איש, מיגו דאיתוסף איסור לגבי עלמא ⁠– איתוסף נמי לגבי דידיה. הויא לה נדה, מיגו דאיתוסף איסור לגבי בעלה ⁠– איתוסף נמי לגבי דידיה.

If she later married his father’s brother, then since a prohibition is added with regard to the sons of his father’s other brothers, a prohibition is also added with regard to him. If she becomes, or remains, a married woman, since a prohibition is added with regard to every man in the world other than her husband, a prohibition is added with regard to him as well. If she then becomes a menstruating woman, since a prohibition is added with regard to her husband, a prohibition is added with regard to him as well.