The only thing more stressful than picking out a Halloween costume is picking out a Halloween group costume. Everyone has their own thoughts on what the theme should be and if you are ever able to make that decision you will still need to decide what every individual person will be.
The easiest way to handle this is just have a family costume. Have the parents decide and if anyone questions it they can just say, "My house, my rules, my costumes." The discussion normally ends there. Someones gotta make the tough calls for the family.
That might seem like a totalitarian way to deal with Halloween but, hey, it worked out for these families.