How do I talk to Mom about needing someone to live with her?

Her grandson, my nephew, wants to move in to Mom's house with his new wife. He's lived with Mom for the past 5 years but recently married. Mom has made clear that she doesn't want another woman living in the house with her, so he moved out. The house actually belongs to me, with Mom having a life time right to live there. The nephew wants to buy the house, leaving in place her right to live there. He wants to buy it because he wants to make improvements but doesn't want to sink money into a place that he doesn't own.

They are both good people, with Mom's best interest at heart I believe. Mom's starting to need someone there because she's forgetting stuff, not eating well, and other signs of early dementia. She also eats healthier if she has to cook for someone. Of course, soon she'll have to give up cooking as it won't be safe. But for a while, cooking for others actually helps her. In the two weeks since the nephew moved out, she's already showing signs of not eating well.

How do I convince her that this is in her best interest? I've tried to talk to her about it, but she's convinced that they are going to kick her out of her house. I've attempted to explain that the life time right stays in force, no matter who owns the house, but she's not hearing that. She's also hinted that she thinks they are stealing from her, but I've proven that's not true; we found all the items she thought were gone. But she still maintains that they will steal her things.

I don't live nearby, so it's harder for me to do anything. There's plenty of family that does live close.