Best thing you can do is talk to her about the baby now and try and explain as much as possible, if you can visit someone with a baby so she gets used to it. Might be worthwhile getting her her own baby doll if she doesn't already have one so she can do what mummies doing with her baby?
I have a just turned 2 year old soon and a month old daughter. The first 2/3 weeks have been challenging as his behaviour has been testing at times, he has however been very good with his sister and shows her lots of affection. I have found explaining everything to him means he reacts much better - even down to that I'm going to the toilet or having a shower!! I've also tried to spend more time with him in his own and let the baby cry abit before going to her. He is reacting much better to home life now. He also still goes to nursery 1 day a week and has spent some time with his grandparents, his dad specifically has given him a lot of attention, more so than the baby.
I think just expect it to take a little time, everything might click straight away which would be great, but equally it might take a couple of weeks.
Lastly we gave Thomas a present from his sister when he first met her and we also only had the four of us in the house when he met her as we wanted it to be about him.