If you don't want your MIL there, I would tell her now. Yes, she might be hurt, but hey, you're the one giving birth here, so your feelings and level of comfort must come first. When I was pg with my first, I had asked my best friend to be there for the birth, but then as it got closer, I changed my mind and really wanted it to be just me and dh. I was nervous about it, but told her how I was feeling, and it was really no big deal. I realize things might be a bit more diffficult since it's your MIL, but I think it's very important that you surround yourself with people who will be supportive and with whom you feel very, very comfortable.