These concerns lead many parents who are expecting their second baby to want to prepare their child for a new sibling before birth. This is, of course, a good idea. Reading books about pregnancy and babies suitable for children your kid's age is wonderful. Talking about how babies need their mom a lot, how they will only be eating, sleeping and pooping for a while, and mentioning that newborns wake a lot in the night... all of those are good ideas. You may even consider having your older child at your labor and delivery.
But before you think it is all about preparation, you should also consider that all of these stresses are yours, and not your child's. Maybe you'll feel a bit better about how your older child will like life with a new baby if I tell you that I had all the same worries, but totally unnecessarily. The transition from one to two children was much easier than going from zero to one for us, at least. My older daughter loved her new baby brother instantly, and was never jealous while my younger child was almost always in my lap or in a baby carrier strapped to my chest, he was tiny and slept a lot. He didn't take away from the time we spent with our older child at all, and I think she hardly even noticed him when he was in my arms.