Appropriate attire for a wedding? And an appropriate gift?

Geesh. I probably wouldn't even bother going from how this cousin sounds. I don't really understand why your mother and aunt are telling you what you should or shouldn't wear; is it possible to obtain a fancy scarf or throw to make your dress more dressy? Otherwise, unless they are going to pay for a new dress, I wouldn't argue with them about it. And as far a gift goes, just because you're invited doesn't mean you are obligated to give a gift. If chocolates are what you would like to give, then by all means go ahead!