We know that this neighbor always takes his car (sometimes without permission). He lives on the other side of the half double that my uncle owns. We have given our uncle advice, but he is not taking it. He is in his upper 80's. The attic is not closed off, so the neighbor a single male can get access to my uncles side
There are liability issues here as well. If your uncle is the sole insured and the neighbor has an accident, I doubt if the insurance company would foot the repairs for an unauthorized driver. In fact there wouldn't even be coverage for such an event.
The insurance carrier may even cancel your uncle's policy if it becomes aware of his dementia and habit of allowing someone to drive his car.
If there's a severe injury and the injured party sues, your uncle would be financially devastated by having to pay the costs of defense himself.
It sounds like your uncle acquiesces in allowing his neighbor privileges which shouldn't be extended. Is he on any medication, has he granted anyone durable power of attorney, does he even still drive himself? Is he still able to understand the consequences of these risky decisions?
I'm not clear about the neighbor having access to your uncle's side of the duplex. Is he coming through the attic and into your uncle's private space to get keys to the car, or does he have his own set? What else is happening when he comes through the attic? Are there issues of financial abuse?
This is a tough situation unless you potentially want to take some legal action to protect your uncle, which may eventually be necessary. Hopefully others will have better suggestions than I.
Why would you think the man might cross the attic? Is there something else going on, or is this because of the suspicion that's been raised by his behavior?
Your brother might want to have someone with him if he does serve the notice himself. And you might want to take your uncle elsewhere to stay for awhile until the tenant is gone.... just to be safe.
Good luck, and tell your relatives to be sure to stay safe. Never can tell what this guy will do.