There have been some studies of this using Y-DNA but I'm not sure if anyone's done a large study of autosomal markers among Sicilians.

In any case, using Y-DNA as a rough record of genetic history, it would appear that the Greeks probably had the largest genetic impact in Sicily after the "native" "Italians" (the Indo-European speaking peoples weren't "native", though they were there earlier than the Greeks, nor were they "Italians" in any homogeneous sense). This is what you would expect, since the Greeks colonized the area rather extensively, and quite early on.

The Arabs and Normans both left their marks as well -- this paper puts the North African contribution at around 6%.

This illustration gives you a rough idea of how common some of these differences are with the rest of Italy. It's a bit of an oversimplification, but I think it would be accurate to say that Sicilians share a lot in common with other southern Italians, both in terms of their genetic history and even their language. The Sicilian language, for example, has dialects in Apulia and Calabria.