If he is mentally incapacitated and needs a guardian, you can ask a court to
declare him legally incapacitated, appoint a guardian which could be you,
your brother, a professional guardian, or other person. His wife could be
guardian if she is present to serve as guardian, otherwise a guardian will
have to be someone who can serve the needs of your uncle. If he is not
mentally incapacitated, then he can make the decisions himself. So he has
to be evaluated as to his mental soundness and ability to make his decisions.
Probate courts handle these issues frequently.