There's a rivalry between every region in Italy. The thing that I've always hated was when people ask if someone is sicilian or italian. You never see anyone ask if someone is neapolitan or italian. But anyway, there's more of a rivalry between southern italy than northern. But I've never seen that much of a difference between Sicily and other southern regions of Italy. People always say there's a huge difference but I've been to Italy and I've lived in America all my life and I've never seen a difference between most southern italian Americans whether they're neapolitan, calabrese, or sicilian.
The reason people separate sicilians with italians is because sicily is it's own island. It's separate from Italy and yes there is a HUGE difference in people, culture, food, attitude between sicilians and the mainland. You haven't spent enough time (or any time) in Sicily to understand it and you won't understand it from simply knowing Italian-Americans or Sicilian-Americans in this country.
