How does one deal with a toxic family member who is harming my mother, who has mild dementia, with lies and deception?

The sister probably has dementia herself.........which is exactly what it sounds like. I don't know that you have ANY real recourse here, frankly. If your mother wants to talk to her sister, what's to stop her? You can change your phone number and not let your Aunt know the new one, but that may upset your mother. Twisted relationships between people can't usually be fixed and if you try to do that, YOU wind up to be The Bad Guy.

If you wind up moving mother to an ALF Memory Care, what's to stop her sister from calling her THERE? Having POA isn't going to gain you any headway on this particular subject either, I'm afraid. Nor will the police........what can they do, exactly? If you have lots of threatening texts, why that could be useful. But if all you have are texts saying that you are a drug addict, or trying to poison your mother, that's not grounds for any criminal action against your Aunt. In fact, that may get YOU investigated instead.

It's going to be up to your mother to say No More with her sister. She will have to reach the point where SHE doesn't want to hear from her anymore before she stops taking her calls or agrees to block her number. I really think that's the truth here, like it or not.

Wishing you the best of luck!

How do you feel about blocking her number? Would she cause more problems by calling the police or something rash?

What about telling her a white lie, like your mom’s phone is broken and she can call you if she needs to check on your mom. That should stop her from calling!

If your mother has only mild dementia, this would be a great time to arrange to get power of attorney (POA) for her and begin to handle her finances for her. Otherwise, her sister may take over and you sound unsure whether this sister has your mother's best interests at heart. You will need an elder attorney to get the official POA form signed for your state. Try to see and speak with your mother regularly, act reassuringly about her possessions and show that you are not interested in taking her things. Does the sister have mental illness? These situations are difficult and there may be a limit to what you can do if your mom does not choose to give you POA and prefers to get care and help from her sister, unless the sister is really abusive or taking advantage of her in some way. Another solution might be to convince/tell Mom she should move to a care facility like ALF where they also have Memory Care which she may need in the future - some are available for low income and some are only private pay. Then sister will be less involved and your mom will get care from others.

Does your mother have dementia? Is she in the care of her sister? Does her sister have a POA to act for your mother?