How do I report financial elder abuse without my name getting involved?

Whoever has a Living Will with the doctor papers that tells the principal is incapable of making decisions. Whoever the principal has listed as Durable Power Of Attorney responsible is for all Finance.

You are the representative payee for X [who? What relation is he to you?].
X has short-term memory loss and is a trusting soul.
You receive X's social security into one bank account.
X's pension is paid into another bank account, to which his niece has access.
You say you have no control over this "pension bank account." None? How did the money withdrawal come to your attention?
Niece took money from his pension account. She states that she did this with your approval, and that the money was in recognition of, or to pay her for, caregiving/support which she provides for her uncle. You say: hooey, you told her no such thing.

Well. There's the misunderstanding. It's hard to see how quite such a big misunderstanding could have arisen, but it's amazing how different two people's view of the same conversation can be.

It seems that the cat is already out of the bag and among the pigeons; and that being so, why do you still feel the need to keep your name out of any report to APS or to the social security people?

You confronted the person doing it. Why do you care if you're anonymous when you report it? They're going to know it was you.

Call Adult Protective Services in your area and report it. Do the right thing.

If you're the payee for the social security payments and also supply this family member with what they need along with care, you're in charge of this person and their money. Why do they have access to their money and why does some other relative with dementia also have it?
Set up a new bank account, or better still get someone else to handle the POA responsibilities because you're not doing so well with it. If the person has a lawyer talk with them. If not the probate court in the town your relative lives in will know how best to advice you as to getting someone else to take over the POA duties.
As for there being financial elder abuse, if one elderly person with dementia is spending the money of another elderly person with dementia, neither is guilty of elder abuse. Since neither are in their right minds.
The one who was in charge of the elderly person with dementia and their money is the one guilty of financial elder abuse because that's neglect.

I thought a payee had to set up a separate acct for the SS check. Also, the person you are payee for cannot have access to the acct. Thats why ur a payee.

Do you or does the family member that is being financially abused have a lawyer? If so contact the lawyer and discuss the matter with him or her.
Since the lawyer is working in the best interest of the abused elder they are also obligated to aid.
Since you are Rep Payee you are obligated as well.
I would contact APS and report the elder abuse. During the investigation you will not be named.

The fact that you are his SS Payee does place a significant responsibility on you.

Is there also a financial or broad POA who also has responsibility for his person’s funds? If so, is THAT person trustworthy?

I’m not sure, but I wonder if it would be possible to get information about your situation from a “consumer fraud” or “elder fraud” officer working with the police department in your BIL’S community?

You will certainly need to protect both BIL’S interests AND also your own.

He is fortunate to have you in his corner.

You could call APS, they will not tell the subject of the investigation who initiated it, but there may be a decent chance you would be suspected by your family member anyway. How much money is involved and how did this family member access it? I think it makes a difference on how to proceed. If you are the rep payee for SS are you saying your bank account was pilfered?