I just recently came out to everyone I know. It caused a few fights with some family members, but the transparency is priceless.

Why do you even need to come out to a kid? Just be yourself. I mean, she's 11 -- there can't be a period of time in which she explicitly thought you were straight and is thereby going to be shocked by this development. Of all people, I'd think a kid sister would be easiest to be like, "Oh, by the way, I'm gay. Now give me my sweater back, you dweeb" to.