Parashas Acharei Mos · Passuk 18:11
Chabad.org — English translation + Rashi
Torah Temima points to Makkos 5b.
English: “She is your sister” — teaches that we do not derive a warning (azharah) by logical inference alone.
No azharah from din
Even an obvious case must have an explicit scriptural warning; punishment and warning are not generated by kal va-chomer.