Mother in law dilemma!!!

Kirsten
My MIL and I have a good relationship and she's very giving, however, she causes my husband and I a lot of stress (not our relationship, but each of us on our own)! She is very overwhelming, knows how to push HIS buttons, doesn't back off, doesn't catch the drift, she's just too much. 
She has asked several times to be in the delivery room to witness the birth, this is our first, first grand baby and first great grandbaby. She also adopted her children (my husband and sister in law) so she claims she's always wanted to SEE a birth. In order to make things easier we just said that we were going to do this alone, just hubby and I, with our immediate family waiting in the lobby for the birth, but I am having strong feelings that I want my mom in the room with us. My hubby says whatever I want he will support and make happen, but I just KNOW that if I have my mom and not his that my MIL will be devastated and offended. She takes things to heart, gets very sensitive and upset and then I wind up feeling bad. However, I want a calming presence in there with me and my mom and husband are the first people I go to for everything.
Husband says I just have to be firm, mean and blunt and tell her I didn't want her in there but I don't know if I can be those things!! What do I do?! So I spring it on her at the time of the birth that I want my mom to come in or do I prepare her before hand and tell her I changed my mind about no one in the room and I want my mom to be there for labor/pushing. I don't know what to do!!!!