Anyone who is going to be at the rehersal should be at the dinner - it's stated in etiquette books. I would find out if it's a monetary thing - if it is - ask if having a less expensive dinner would do the trick. You want everyone there and she only wants to pay $X, then figure out together how to both get what you need. I've been to rehersals that are in the family's back garden with simple catering (mexican food, bbq) and if you put some effort in, it can be really cool - decorating and such. The money part and the guest list are the worst parts - I promise. It's all down hill from here