What do you do when your sibling has tried to gain total control of your parents finances?

He has hired a lawyer that represents himself and secretly obtained POA from your father with dementia without your knowledge or concsent and may have changed trust documents. He has hidden all legal documents from your parents home so you have no proof that anything has occured. He does not answer phone calls or e mails and your parents deny that he has done anything wrong.
Nor can they remember any transactions that he has them make. You have no idea what you are in for upon their death or time of any one of them being incapacitated. He has already taken care of your Aunt's estate that was living with your parents upon which he withdrew large amounts of money from your parents estate to "pay her expenses" until he could "open" her estate". In the meantime, her social security checks, pension, and other assests remianed
unaccounted for, care provideer claims that he has not been providing for her
needs aside from 1 months payment so far. What's going to happen to his parents. As the only other sibling, I feel, left out, betrayed, and robbed of my rights to make decisions and participate in my parent's care and safety.