What are steps to set up legal advocacy for my brother whose spouse died a year ago?

Dependent on mentation he can appoint you his POA. He doesn't need to be mentally perfect to do so, but he needs to be able to know what he is doing. He would be examined for competency by the lawyer; many will do housecalls. You would not be with him when examined but could be there. If you wished to hire a fiduciary you could do so as his POA, to pay bills, and etc. They generally in California cost about 90.00 an hour. Dependent on estate and bills several hours a month may be needed.
If your Brother is not now competent to make the decision, and has no near relatives (children?) who wish to be his conservator, you could apply to be his guardian, or request that the Court in your State assign a conservator for him; see an elder law attorney for this.

As said a conservator will need to be appointed.

Judith, how is it that others have access to his accounts?

Has he told you this, and have you seen evidence of it yourself?

Is it possible that he has automatic payments coming out of his accounts that he doesn't recall?

Does he want the help? Is he incapacitated in any way? If not, he has the right to make his own decisions, no matter how bad others think they are.

If incapacitated then a conservator could be appointed through a court process. Check with an elder law attorney.

Bumping this up.

Best wishes.