Totally with Canuck. Can't believe you're even entertaining the idea of letting a 5 yo's tantrums dictate your adult relationship.
Make sure you're not taking her to McDonald's or for a "treat" when you take her out of the house when acting up. Otherwise you're teaching her that treating the fiancé badly gets rewarded rather than punished. Sure, remove her from the situation to save the fiancé grief, but take her and sit in the car, or some other mundane activity, then take away a favourite privilege when you return to really drive the point home.
It's almost impossible to get 5 yo's to understand logic, but they sure do learn cause and effect real fast (if you stick with it)