If three of four adult children suspect there has been some in-appropriate movement of financial assets between parent and the fourth child who has the power to get access to bank records?
With a will, you can go to a lawyer.
if you suspect fraud, then take your case to the police. if it is just a family spat, then you are out of luck.
Unless the parent has signed a power of attorney over to one or more of the children, only the parent can give access. IF the parent is no longer mentally competent to be managing his/her own affairs, you can get a doctor (or two- it depends on your state law) to certify that, and then the court will appoint a guardian with power of attorney- that would usually be the person appointed as executor of the will if the parent has a will.
If the person, the adult parent, is in sound mind he/she can transfer funds to anyone he/she chooses. If the person with the FINANCIAL power of attorney is misuing the power any adult can go to Court and challenge the person who holds the P/A. There is no legal action against brainwashing. The LEGAL assumption is that the person who signed the P/A was in good mental health, capable of understanding the P/A, capable of signing it. Immoral has nothing to do with the law. Unhappy and believe fraud is being committed? LEGALLY challenge the P/A.