Q: If my older brother says he is power of attorney for my mother who has dementia doesn’t he have to prove that to me?

A: Yes, he should show you proof of his authority as deputy under the power of attorney. If your mother is suffering from dementia and has been diagnosed, the power may not be valid if executed when you mother was not considered to be capable of executing legal documents. You should speak to an estate attorney for guidance.

A: Start off with the presumption that your brother cannot evict you.

I presume that you were living in the house with your mother for the past 38 years; during which time, your mother never evicted nor sought to evict you from the home.

I presume also that now that your mom has dementia, your brother has taken control of her medical care, her finances, and the payment of her expenses.

As to your brother's "control" over your mother's affairs, the real question is what type of authority did your mother give to your brother and when was it given to him? Your mother may have given your brother the power of authority over her medical decisions (health care proxy) or authority over the payment of her expenses but unless he can show you that he was given an expansive power of authority over all aspects of decision-making for your mother including the authority to pursue legal action on her behalf (the eviction action), he cannot take any legal action against you. My suggestion though is that you and he need to sit down with a 3rd party to talk through the issues outstanding before they blow up and sever family relations.