Yeah; for the most part, HRE let's you alone except for raids (small armies attacking towns) that are easily handled once their lone Italian city falls. Sicily will keep attacking if you own the southern city of northern Italy (which may be Bologna or Florence; I don't remember). I found that the easiest thing to do to neuter Sicily while I concentrate on the Byzantines is to take Palermo by sea. It's generally not defended well. I never could stick to a plan, though. Too many variable with Venice.