My fiance's grandmother, who has dementia, is telling lies about me. What can I do to protect myself legally?

I have moved in with my fiance and taken over the care of his grandmother who lives with him and has dementia. From what I've been told, she's always been a very difficult and just plain mean woman and a pathological liar, but this is becoming worse as the dementia worsens. In the short time she has been under my care, she has already told neighbors that I lock her in her bedroom when I have company, that I am "kicking her out on the street," I am hiding her food, stealing from her, beating her dogs, etc. Of course these things are completely untrue and most people know this, we live in a small town and her reputation precedes her. My concern is that someone will believe these stories she is telling and the possible legal actions that could be taken against me. Of course, we have tried talking to her about telling stories, but she just denies telling the stories. Any advice?