I dont want to be controversial here, but whilst i have heard far worse accents than De Niro's (Pacino's for instance), his accent is unmistakably american. The stresses he places on the words, are correct, and grammatically im sure he's got it pretty spot on since that's just about repetition, but accent wise he sounds no where near native. This is perhaps partly why he speaks even more softly in the italian dialogue in an attempt to mask his accent

Im sure Frank Sivero is referring to De Niro's ability to speak the language (learn the sicilian dialogue) rather than the authenticity of his accent - and furthermore it's not like he is going to criticise his colleague publicly.

Re one of the above posts that points out a grammatical error with the use of the 1st person ("eu"/"iu") rather than the 3rd person singular ("he"), i've never heard "iddru", perhaps it's a regional variation, but the standard sicilian word for "he" is "iddu" - and owing to De Niro's whispering is easily muffled into "iu".

Granted it takes a little bit of a musical ear, and/or familiarity with the language, but just listening to De Niro's accent compared to Sivero's, or Don Tommasino's should make it painfully clear that he sounds very very different!