Don Cardi -

I haven't read up on Capone in a long time either, but I recall hearing/reading that he wasn't a member of the Mafia because of the reason you stated (he wasn't Sicilian).

I think Mario Puzo hinted at this in "The Godfather" when he wrote about Vito sending Capone a message that he should stay out of the battle between Sicilians (Vito and Maranzano).