What is the best way to comfort my Mom with Alzheimer's who is so sure that someone is in the house at night and are talking loud?

bullyboy, does your Mom wear a hearing aid? I remember my Mom when she got her new hearing aid was hearing what sounded like a talk radio station. I knew exactly what Mom was talking about because I was getting that same thing with the landline telephone. And it happened very late in the afternoon and sometimes into the evening. It's was a telecommunication hiccup somewhere. It has since stopped.

Or it could be a stage of Alzheimer's that your Mom is experiencing.

Could be something as simple as the TV is on loudly. Could be she is having hallucinations. This is something to tell the doctor.

Maybe I misunderstood what you wrote, but it sounds like you are saying your sister leaves her alone, maybe at night? Maybe Mom is afraid of being alone and could use a sitter for those times.

Take her to see her geriatric specialist. And have your sister watch Teepa Snow videos.

Better drugs. See the MD asap