The authorities are not harassing people when they investigate them. Just because YOU know that you are a good parent does not mean that they know it. Most bad people look and publically act perfectly normally--you would have no idea that in private they are abusive monsters. So, don't take it personally that you have been investigated. Wombat has some very good advise. Also, I would stop allowing the suspected family member into your home for any reason, and would certainly recommend avoiding him/her. Are you sure your FIL is not involved?