How can I get her to understand?

I have a 2 year old daughter that will be 3 in September. She is very smart. I have been trying to let her know that (mommy is going to have a baby.) I think she knows a little bit b/c she touches my belly and says "Mommy has a baby in there." But, how do I make sure she really understands that? My sister has a 5 month old son and also a 1 1/2 year old as well. Soo my daughter has seen them both when they were babies. She did go through some stages where she was a little jealous but it all worked out. My poor 5 month old nephew had to go through both of the kids bothering him.lol I wonder is that how it’s going to be with the new addition in the family? But they are all the closest little cousins I have ever seen. 🙂