Crazy soon to be MIL. Advice needed!

So my fiancés mom is well, bat shit crazy. I get a long with her, but sometimes she can reallyyyyy press my buttons. The other day she informed me that it would be her, my fiancé, and my mom in the delivery room. Ummmm excuse me??! She just invited herself right on in!! There is no way in hell I want that negative woman near me when I'm giving birth. How do I fix this situation?? And not be a total bitch about it? It's her 3rd grandchild so it's not like it's totally special being the first. Thanks in advance!