Margy is right. That is unacceptable behaviour. I would also put locks on ur tv and computer, kids can see anything nowadays. Also ask her where she learned what she was doing. It could have happend to her, she could have seen it, or a friend could have told her. Either way she shouldn't be experimenting with her 6 year old brother. I'd also tell her its ok to explore her own body as long as she's the only one doing it. Most kids bodies are changing at that age, and masturbation is normal. But, it is all up to whether u allow her to do that. Good luck. Hope this helps.
No this isnt just play this is an older child misbehaving...you should speak very firmly to her that this isnt a good thing.Supervision when she is with smaller children is what I would advise, she is at that age where sexuality is rearing its head so time to tell her the facts, and make sure this behavior is nipped in the bud. Tell your son that it is not alright and that you have talked to her ....