"My brother will help you with the luggage", 'could' be expressing 'willingness': the fact that he's happy to do this for you. It could also simply be expressing a future event. It depends on the context. "Will", used to express 'willingness' is most commonly found in the "I" form of the verb... e.g. "My bag is so heavy!" "Oh, give it to me. I'll help you with it." "I'm going to be late for work!" "Don't worry, I'll give you a lift."

I would tend to say none of them, except B. expresses the opposite of willingness. Oddly enough, the verb WILL often has nothing to do with being WILLING, (i.e. agreeable to something). It seems to mostly be used as an auxiallary verb.