Baby Mama Drama

Was wondering if anyone had any advice on how to deal with a baby mama. I am engaged to my fiance and he has 2 wonderful boys from a lady who is a bit immature. We are constantly arguing and he tends to take her side, he says he doesn't want to start a fight with her yet he is perfectly fine upsetting me and arguing with me? It gets frustrating at times because I feel like he is always going to stick up for her, I don't ask for anything the only thing I ask is to be included I am going to be a stepparent so I think it's only fair to include me in decisions. Is that too much to ask for? Am I being unreasonable?