Paranoia, and angry mother. Any advice?

Starting nearly two years ago, my mother believed her nephew and his wife were stealing from her, messing up documents and stealing some, and bringing in some new things into her apt. My cousins and I all tried not to confront her, things went along shakily, and she moved into an independent living place. She still believes her nephew and niece do things to sabotage her, and gets angry if anyone hints that she is mistaken. She's pretty 'with it' otherwise, and passes dementia screenings every time. Need hints on how to get to the point coming up - that she'll acknowledge memory problems. I'm her only child. Today I rattled her by telling her about 2 incidents she had told me yesterday, but could not remember she'd already told me. That led to talking about all the past troubles. I gave up talking to her Drs. last year. She believed her neurologist made fun of her.