Hi Beckii,
Well, the chilli plant is directly related to the tomato and the potato. It's easy to see the similarity with the tomato, but potato? I'm not sure there's any resemblance visually.. It might be worth putting this question in Home & Garden to ask about Plant families etc , someone will give you exhaustive information about the relation with other plants/food.
Certainly you should include tomatoes in your project. And another angle you could use is the fact that chillies are used as a spice, so collect examples of other spice foods in their raw state, because most spices are dried and powdered. eg. garlic and ginger are really easy to get hold of, then things like pepper, or cloves. etc. etc.
I hope this is a bit of help, and it's just my opinion..
cheers
bill