Try to serve foods that are easy to eat with fingers so people aren't struggling with a knife and fork. Unless you're having a sit down dinner, most people will be standing and eating which can be hard when you have to cut something. I'm hosting a grad party for my daughter this week and we are having hot and cold finger foods(chicken tenders, cheese and crackers, crudites, beef crostini, fried shrimp, asparagus purses, and dessert pastries). For the drinks, I'm setting up a serve yourself bar with water, soda, beer and wine.
It depends on where it's going to be, how much money you have to spend on it and how many people you'll be feeding. Most graduation parties are pretty informal outdoor picnic-like events. If that's the case, keep it simple. You probably won't have a way to keep anything hot or cold for long so serve stuff like fried chicken, dips, fruit salad and cook burgers on a grill. If it's a more formal indoor event, have it catered. Most caterers let you taste their food, so just pick whatever you or your sister likes.