Have her in the wedding party, just for his family's sake. It won't be necessarily that you've picked her, more like you have to. When my hubby and I got married I barely knew his sister and would never have asked her, but I was told by a million people it was proper etiquette to ask her. (well they are very close so it would have been rude but it just never occurred to me to have to ask someone I didn't know well)

Also, think of it this way... you won't have to plan the wedding "with" her. You plan your wedding and you TELL her, this is the dress, these are the shoes, this is how you'll wear your hair, this is what time the limo is coming... etc. You get to call the shots. If she tries to act like a brat her father will have no choice but to see what she's doing.

Its called "keeping the peace" - and it seems like you've been doing it as much a possible. But its very hard to have a happy marriage when you hate your in-laws and the other way around.

Someone once told me, "As long as you do the right thing, you don't have to worry about what anyone else does."

Besides, if you ask her and she says no, who will look bad then?