Due to the age of your niece and the lack of previous contacts by the biological parent, supervised visitation should be requested and is usually granted until a routine has been established. As you have discovered, you can not prevent the visits from taking place, but you can cooperate in a way that can attempt to protect your niece. There are many agencies that can advocate for her and/or provide a place for visitation to take place where the interaction can be observed and documented. Also, more importantly, your niece is not too young to learn about proper touching, how to communicate if something were to occur and have someplace to allow this communication to take place that is positively proactive. Now is the time to invest in finding the resources for her.