You were young, so was he, it is really no-ones fault. The mean girl could have been molested or introduced to sex somehow at her age, making her act outwards via all of the other kids. I would not even bother to bring it up to your brother either, it does not seem to have played that big of a roll in his development and if he is happy, why bother? I think you need to forgive yourself, because healing starts inward and there is nothing you can do about the past.