Nowhere does it say that Carlo was a made man. And, even if he were, it would have been considered a family matter, not subject to some Mob BS rule that probably wasn't enforced anyway. Besides, Sonny was acting Don--and who was going to say boo to him?
I think I guessed that he was a made guy due to Michael moving him up once he became boss. He must have been made sometime between Sonny’s murder and Michael becoming don. Or as you said, it was a family matter and Carlo had no respect.
