As an Italian (but not Sicilian), I'd say that the portrayal of Sicilian customs is very accurate, especially in the Sicilian settings. As for the language, I can just say that DeNiro's almost completely Italian acting in GF2 was absolutely great, if you consider that he is NOT mother tongue. The occasional Italian words one can hear in the trilogy by non Italian speakers sound very "American" (if you know what I mean) but I presume sounding very Italian (or Sicilian) would not be realistic.

I don't want realism. I want magic! Yes, yes, magic. I try to give that to people. I do misrepresent things. I don't tell the truth. I tell what ought to be truth (Blanche/A streetcar named desire)