Sicilian has really a quite amazing history, one fairly different from the rest of the romance languages in the modern Italian Republic. I made a YouTube video about it that does a great job explaining. In short, prior to the Norman Conquest, Sicilians generally spoke Greek and Siculo-Arabic (which has become Maltese).

The current Latinisation of Sicily only really begins in the 12th century or so. Hundreds of years after the fall of the Western Roman Empire. The Sicilian language comes from a very interesting mixture of the languages that were imported by settlers brought by the Normans, neighbouring areas and the pre-existing Greek, Arabic varieties and maybe even some surviving Latin dialects.