as far as I know the rivalry is due to the fact that before the unification of Italy, in the Kingdom of the Two Sicilies, Naples was the capital and the King Borbone, as it was of Spanish origin, was more Neapolitan of the Neapolitans, of consequently despised Sicily, which did not even have a detailed map of the island, there had never set foot, and there are various governors who succeeded have done what they wanted in Sicily, without the king said nothing.
When first with the Neapolitan Republic and later with the French invasion the Bourbons fled in Sicily, defined savages, the Sicilian people, corrupt the clergy and the nobles and did not see the time to return to Naples.
but is a subjective fact, my cousin has got married to a Sicilian from the city of Ribera, he is a good person and also his family is made up of good people, his sister even dedicated poetry after the wedding.