I thought my MIL was going to be bad -- she even made me cry the night of my rehearsal dinner. My husband was an only child, and they were very close. So much so that when he and I first started dating in college, he talked to her on the phone practially every day (I, on the other hand, spoke to my mother once every two weeks, or as little as possible).
Once we were married, it wasn't as bad as I thought it was going to be. Now, he hardly ever speaks to her -- although part of that could be that she moved to the beach to live w/ her boyfriend (another long story). I asked him just last night when was the last time he heard from his mother, and he couldn't tell me.
KR does bring up the most important part: as long as your fiancee is siding w/ you and not his mother, you're golden.

Things should get a little bit better once you're married and she realizes that she doesn't control him as much as she thinks she does.
I also wish you good luck!