TBH as much as we prepare them and explain and show...it's a pretty complex process and I don't think most toddlers give a hoot. Little girls seem to be more into it (from my own experience only) but it really doesn't take long before they can't ever remember their sibling not being there.
My DS is similar and he'll be 3. He gets that the baby moves, and we've bought it nappies (cos he doesn't wear them any more), and furniture (we weren't having any babies after him so had to start our collection again lol), but in reality he can't understand that a real live child will come from me and be his sibling, and won't until he's much bigger.