How does one change guardian?

I am a caregiver to a 78 year old man who had a stroke a little over a year ago and his niece became his guardian because he needs he can't be myself. He is disappointed and mistreated by his niece. She make sure all his material needs are met but that's about it she has no compassion she is mean to him and his quality of life is nonexistent. She keeps everything from him he has no idea of his finances, the will was changed and he has no idea what it says now, he sits in a recliner day in and day out with no contact with the outside world. She will not let me do anything for him besides take care of him in the house. she goes to doctor's appointments that were made ahead by the doctor in other words if he wants to go see a dermatologist or he wants to go see a doctor about his knee or anything like that it doesn't happen. And she is mean to him she belittles him and it's kind of rough with him at times and will like I said will not let him have his wallet I will not let him go into his