I have been falsely accused 4 times for abusing my nephew. How can I stop this and prevent this from occurring in the future?

He is the son of my brother, and the woman who keeps falsely accusing me (they are not together). Fall 2013 the woman called Children and Youth (CaY) to declare a sexual abuse charge. This eventually became unfounded. Winter 2015 the same woman called CaY again to claim the same incident from 2013 had occurred. This again went unfounded. A few weeks ago she tried to file the same complaint; however, this was dropped due to being within the 1 year grace period. This weekend, a new allegation was issued to CaY. This time the allegation involved a gun and no sexual abuse, therefore, a new investigation began. She recently posted very negative comments and false allegations on "Facebook" about myself and Children and Youth. Also, she texted my brother what I am considering a threat on my life.