S.G.
I'm happy to hear that your sister wants to find out who exposed her son to such behaviors. Many parents go into denial, even when presented with the truth.
I would call the Children's Services Department in Clermont Co. to ask them for therapists they know has experience in this area and how to handle it. It will most likely turn into an investigation and she wouldn't want inexperienced or too many people asking questions or possibly scaring the child or change his story so she can find out the facts.
Good luck! I've been through this with a relative. The mother went into denial though. It was unfortunate but in the end, the molester was removed from the children's lives. It was the mother's boyfriend.