Hi P.,
Our first son, Thomas, was just 2-years old when our second son was born, and I had exactly the same concerns as you do. Throughout the pregnancy we made sure Thomas knew that this was a going to be a BIG adventure for all of us. Whenever we talked about the baby we kept it very upbeat and exciting. Thomas helped us get the nursery ready and was with us on all of our shopping trips for baby stuff.
Although it was not how we'd planned it, Thomas had to be with us at the hospital when Sam was born, (everything happened incredibly fast and we didn't have time to wait for my MIL to arrive to stay at home with Thomas). In retrospect we think this was very, very positive for Thomas. He got to see 'Baby Sam' almost as soon as he was born and felt very involved with the whole thing.
My husband was able to stay home for the first week after Sam was born, which made a huge difference as he was able to dedicate a great deal of attention to Thomas. We have always, from the word go, let Thomas touch Sam, including touching his face and head, as long as he's gentle. Initially he wasn't too interested in 'helping' with diaper changes or bottles, but we always asked him if he wanted to, and he certainly does now!
One of the best pieces of advice I received was from the pediatrician. He also has two children just two-years apart and he told us that for the first few weeks whenever we were in a situation where both Thomas and the baby were demanding our attention at the same time, we should, (whenever it was safe, obviously), respond to Thomas first not Sam. Of course it's difficult when the baby is crying, but if you know the baby is safe and does not actually need you at that precise moment then the baby will be absolutely fine, and the older child learns that they are not going to be pushed aside. I only had to do this for a couple of days, after which Thomas stopped demanding my attention so much.
The fact that you are posting this question shows that you are a very caring, dedicated mom, so I don't think you are going to have too many problems. At the end of the day, you can only do what you think is best. It's a learning curve for everyone involved, and your daughter will let you know when something is not working for her.
Good luck, and very best wishes for a smooth delivery. This is such an exciting time for you all!
D.