I actually created a melting pot mod for Sicily ("An Offer You Can't Refuse, or AOYCR) a while back, if you want to check it out. The link's in my sig, but it's been incorporated into most of the major mods in some form or another, too.
The way I handled Sicily was to have 3 cultures all localized to Sicilian, but located in different culture groups; the culture which formed depended on the league. Therefore Siculo-Arabic, Sicilian (formed from Normans or Italians, IIRC), and Siculo-Greek. Some of the mods which incorporated mine changed the localizations though; I'm pretty sure Siculo-Greek was named Griko, as suggested above.
Didn't do anything with Normans and Italians, though, so that'd be an area which could use some attention for sure!