when DD2 was born DD1 was 26months. DD2 bought her a present that she got when we came home from hospital which was her own baby, a bed for baby, nappies, bottle dummy etc. She was more excited than anything at first and wanted to "help" with EVERYTHING so if i was doing something where her "help" would have been really a problem (ie. changing a poo-splosion) i'd tell her to do her baby.
It was after the first week and she realised DD2 was here to stay that she started getting a little bit whiny and needy. I went with Nae's approach of not scolding for being rough (unless it was excessive. She clawed DD2's face one day while i had my back turned for a few seconds. But i spoke calmly because i think yelling/punishing would just maake her resent bub more) and not making a huge deal when she had a tantrum.
I think the most important thing is to keep them included. When DD2 went down for naps we would play/read/bake or even just snuggle just do something special, just the 2 of you. It's hard at times but you get into the swing of things pretty quick. In saying that though i didn't have a c-section. Good luck, i'm sure you will be fine once bub's here