Pregnant and expecting problems with fiance's ex, look for advice?

My fiance and I have been together for about about 2 years, we are in our 20's, and I am three months pregnant with my first. He has a 6 year old girl and a difficult selfish ex. She make problems for him when he is just interested in seeing his kid. She still wants to fight about stupid thing like when they were together but they have not been together for 4 years. I pretty much stay out of there business, its his problem. But now I am pregnant, I know she is going to be jealous and have a big problem with it. I expect her to drag my innocent child into adult problems like she does with her own. The thought of this make me completely freak out and I don't know what I will do to her. So now that we all know the reactions that will happen, I ask how and when should she find out? I am due in January and his daughter will spend the month of July with us and Christmas. I was thinking not to tell the child in July because I am not showing and we don't know what it is. But I don't think she should be left out of it, but then again she will run and tell her mom and drama will result. When she comes for Christmas then she can feel the baby kick, see the room, know what its going to be etc. Do you think we should tell the baby's mama about it, or just tell her daughter her about it? When she comes to my man running her mouth about OUR child should I confront her or let it go? Anyone that has been in this situation I would especially appreciate your responses.