wow, all these sounds so familiar to me. my aunt (my uncle's wife) is exactly like that. she does nothing during the day, but to spend my uncle's money (he's a top executive and earns a lot of money per month).
everytime she comes home...it's a nightmare. she complains about everything, food, christmas presents, everything. this year it was that we didn't celebrate christmas very much, so instead of having a xmas meal we had a normal one. she can't understand that my 80-year grandmother is devastated because her sister had just day a week before and was not very keen on doing anything "happy". plus, she doesn't stop whining about how tired she is (and she only works an hour per day!!).
today we found out she had bought my uncle a new bicicle...with what money? (she hasn't own savings) with the euros my poor grandma gave her daughters for christmas. I remember one day it was the top of the tops. she began to say my grandma hadn't cooked enough food (what she indirectly meant was: this lady is too old and she is not capable anymore, which offend us so much). since I didn't want to eat anymore, I asked her if she wanted to eat my prawns (which I hadn't touch). she said: what? I don't want your nasty leftovers!!! i just had to go, I would have poured my glass of water on her face.
the only thing I can tell you, heather, is that, at least, your fianceé sees whats happening. my uncle doesn't, he is just like blinded by her giant boobs!
good luck, loads of patience and congratulations for your wedding. maybe the key is to act cold to her, putting some distance, and letting her know that the one who rules here is you.