oni, n. 鬼 1. devil. 2. cruel person.
That's what my dictionary has. It also says exactly what mandylion said about onee and onii. I'm sure nobody would notice if you said oni but hehe, it would be embarrasing to call someone a devil so make sure you get that extra い in there. Also I have a question. I know these can be used for brother and sister but don't you also use them when talking to a stranger? I was confused about this and when to use it. どもありがとうございます!