What does your fiance say about his mother's behavior? Has he stood up to her and told her that her tantrums are unacceptable?
Unfortunately, until he stands up and says something--and backs up his words with action--she will continue to run roughshod over the both of you. Its not your job to put her in her place, its his, and the situation will NEVER improve until he says something.
Then again, there are some people that are so far gone around the bend that nothing anyone says will improve their behavior. In that case, I suggest moving far, far, away.
Good luck to you, come what may!
