Reference for Makkot 27:11

ואידך אחותו דסיפא למה לי לחייבו על אחותו בת אביו ובת אמו לומר שאין עונשין מן הדין

And why do the Rabbis need achoso d'seifa? To make one liable for his sister who is his father's daughter and his mother's daughter [his full sister], to teach that we can't derive punishment from a kal v'chomer [specifically that we don't derive it from the prohibitions on both half sisters].

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