I don't really have a problem with anyone eating alot of my food. All my kids and my husband are really thin, as a matter of fact so thin I am tired of hearing everyone tell me I need to feed them more. The oldest one even comes home from school mad because kids ask her if she is anorexic. Because they are so thin they actually try to stay away from my food because they are afraid if they eat what I eat they will lose weight. They do eat my veggies and nuts though, but I don't really mind because it is good for them. I just go back to the store.
I think it would probably be a good idea to hide your foods in you room or somewhere. Even though you live there rent free it is still disrespectful. Like I tell my kids, if you didn't buy it or if it wasn't given to you then it is not yours and you must ask to use it etc. Maybe when they bring up the rent free thing when you ask about your food offer to pay rent. Chances are they won't accept but will realize just how important it is to you.
Nancy, that is hilarious about your brother and the bananas.