For a brother and sister or any boy and girl, go as each other or just switch genders.
One year my mother and my aunt decided instead of buying costumes, that they'd dress me as a girl and my cousin as a boy. They just dressed us in some of each other's clothes, put a baseball cap and glove on her and a wig and make up on me;and we were all set.
It was pretty embarrassing at first, but we ended up getting a ton of candy and I actually had fun!
Thanks to Micheal for this fun idea!