If your aunt offers to bring a pie, take her up on the offer. This is one less item you need to buy or prepare. Asking others to bring a food item is a good idea. Just make sure you give that person plenty of time to prepare it. Prepare only what you need.
Many people prepare more food than is needed.
Keep the Food Pyramid in mind as you plan quantities. Plan to use leftovers. As you write your menu for the big event, plan for the entire week. Extra turkey or ham can be used in many different ways. The problem is both.