Sweet 16 And Combined Graduation Party

My sister is graduating from high school and she wants to have a party. I want to have a Sweet 16 and my b-day is in early September. Our parents said we can have a combined party, but it would have to be in July. I want it to be fancy, but my sister doesn`t. I just want to wear a big fancy dress. Does anyone have any party ideas/suggestions? I need all the help I can get. Plus, I am on a budget of $1500, but I think I can get my parents to increase that. Thanks!