Saying no to the party is the right thing.
Let her know that you are hurt that she wants to go to a party when no one knows when she will be back to visit -- a job could easily interfere with coming on weekends; this is a family event and more than just gramma's b'day party, it is a weekend celebration; the original plan was she was coming to see the family for the weekend, not hanging out with buddies and potential boy friends. If they want, they are welcome to come visit for an hour on Saturday or Sunday before she leaves (?).
So, just remember that she is the one who is not being fair in wanting to go to the party, not you in wanting her to visit at home.
Good luck! You have the right to want her to visit at home.
