I heard this question from Jillian Waldman.
Which one doesn’t belong out of brother, sister, nephew, son?
Sister is the only female one, nephew is the most indirect relationship, and son is the only one to imply your own offspring rather than somebody else’s. Therefore the correct answer is brother: The only one which isn’t obviously a correct answer. It’s different in that it is less different than the others.
Original version, March 2009.
Updated and added here March 2010.