As BarbBrooklyn suggested, your mom's Dr. should be consulted. There may be a medication he/she can prescribe that will give your mom some peace of mind.
And paranoia is a symptom of dementia. When my dad developed dementia due to liver failure at the end of his life he had elaborate fantasies about the staff of his nursing home. Initially, in his fantasies, he knew what was going on (in his mind) at the nursing home and because he was in the know the danger he perceived didn't affect him but his fantasies and paranoia became darker and more threatening and that's when we began having problems. I patiently told him over and over that he was safe and that I'd never, ever let anything happen to him. I listened to him and consoled him and that would pacify him for a while and then it would start all over again. I never told him that his fears were nonsense or tried to convince him that what he was afraid of wasn't real. I just continually tried to reassure him that I would always make sure he was safe.
Of course your brother-in-law isn't out to kill your mom but how would it feel if you knew someone wanted to kill you but no one believed you? That would be terrifying. Reassure your mom that she's safe, that you'll always protect her. And it would be a good idea if the next time your sister visits her husband doesn't join her.
Chleco, I found this excellent article here on Aging Care regarding paranoia. My Dad developed what was called "sundowning", a phase of dementia, thus his reasoning was questionable late in the afternoon and into the evening.
Paranoia is a symptom. It can be a symptom of dementia, or of a UTI or many other things.
A change in mental status should always be reported to the doctor and advice sought.
And paranoia is a symptom of dementia. When my dad developed dementia due to liver failure at the end of his life he had elaborate fantasies about the staff of his nursing home. Initially, in his fantasies, he knew what was going on (in his mind) at the nursing home and because he was in the know the danger he perceived didn't affect him but his fantasies and paranoia became darker and more threatening and that's when we began having problems. I patiently told him over and over that he was safe and that I'd never, ever let anything happen to him. I listened to him and consoled him and that would pacify him for a while and then it would start all over again. I never told him that his fears were nonsense or tried to convince him that what he was afraid of wasn't real. I just continually tried to reassure him that I would always make sure he was safe.
Of course your brother-in-law isn't out to kill your mom but how would it feel if you knew someone wanted to kill you but no one believed you? That would be terrifying. Reassure your mom that she's safe, that you'll always protect her. And it would be a good idea if the next time your sister visits her husband doesn't join her.
https://www.agingcare.com/articles/hallucinations-delusions-and-paranoia-151513.htm