My brother has been paying my mother’s bills. Now he isn’t and he's claiming fraud. What do I do?

You don't do anything. Your brother will get nowhere with any bank. Assuming his name is not on accounts your Mom's money is safe. She should, of course, be paying her own bills, and if she is able (has the funds) and your brother has proof and receipts of his payments for her, then he should be compensated by your Mom or you. You don't say if you are POA for your Mom. You don't say if your Mom is competent in her own finances and decisions. You don't say why the brother paid all these bills without a word, but that is his choice if he told no one. See to it that you don't make the same mistake. Your Mom's assets go to pay your Mom's bills. Hope you will keep us updated. If I were you and your bro is inappropriate you should tell him to present to Mom any bills, receipts etc. that he paid and you and Mom will look them over with an elder law attorney. Try to have as little contact as you can if he is acting inappropriately.
Sadly, when we take on the bills of another, we have no way to get back the money we have paid out.

Sorry, I don't understand your question. He "personally" has been paying her bills? If so, why? Does she not have SS? Where does the fraud come in? Did she have money and he didn't need to do this?

"An md said his bank is going to recoup the money from years of bills."

Who is "md"? Banks do not recoup money. This only happens when your acct has been hacked and they replace money. They do not go back years and look for checks that were written for your mother. I don't think Banks would be able to go that far back. I don't think this is a banks job. That would all be up to your brother to show proof. He would have needed to have kept good records and prove fraud. He will need to sue his mother. Not sure a lawyer would take this on. And if Mom has no money, as they say, can't get blood out of a turnip.

Is there some mental decline with brother?

Just to clarify - was he paying from his own money or was he acting as your mother's POA and paying from her accounts? If he was paying from his own money I don't blame him for stopping the gravy train, but I can't see any basis for a claim that it was fraud unless he has evidence of coercion.