I think people do that out of politeness.IndomitableSam said:I have an identical twin sister - the only thing different about us is that her hair is a few inches longer. People will always ask us "Are you twins?" which isn't so dumb in itself... but there are those rare, stupid few, who go on to ask "are you sisters?"
... No. No, we're not. We're just twins.
I get kind of uncombfortable asking people about their "twin" or if they're "twins" (which I don't because it's pretty damn obvious if people are twins or not). I have a friend who is an identical and I always refer his twin as his brother, he doesn't seem to mind.