K.M.
I would just tell them you have a baby in your tummy, and keep telling them all the time. Make sure they are a part of everything. When you start feeling the baby move let them feel too. Talk about baby all the time so they won't seem so surprised. I know some people who have even packed gifts for the older siblings in their hospital bag. Once the new baby was born they said he/she brought the gifts for the older brothers. My children are 7, 6, 14 months, and 3 1/2 months. We never had a problem introducing new babies into the family. Oh, you might also have shirts made that say something like I'm the Big Brother, and I'm the Biggest brother. They can wear them to visit you in the hospital. Good luck. I'm sure it will be fine. :)