4 year old niece

My 4 year old niece was subjected to sexual assault by another little girl. She told her mom that she licked her down there. The cops were called, the little girl has been sexually abused by her step brother when she visits her dad. The little girl then did this to my niece.

What can we do to help her? She has already taken off her underwear and is touching herself, moving things around saying her friend did it, so it’s okay. We are currently teaching her that it’s NOT okay.

Please help. We are all at a loss on what to do. We want her to “forget” what happened, because we don’t want this trend to continue.

What helped you? What made it worse?