Is there anything you can do personally to revoke the sister’s power or take legal measures in order to effect some curtailment of her abuse of the agency authority?
Sadly, the answer is no. Not directly. Not while acting in your individual capacity.
A power of attorney simply creates an agency relationship. And unless the authority to act on the grantor’s behalf is “coupled with an interest” (where the attorney-in-fact has given consideration for the power) it can be revoked at will. That is, at the pleasure of the maker and for cause or without cause.
In this sense it is similar to an employer/employee relationship.
Consequently, since you father granted the authority, it’s his prerogative to choose whether or not to revoke it.
However, if I were you, I would document all that you have told us. Bundle it up in a neat package (copies that is). Tie a nice pink bow on it and send it to the sister with the message that if she does not immediately surrender her authority and make reimbursement of the embezzled funds, that you will present the package to the police and prosecuting attorney for felony prosecution.
Need I explain why I don’t allow my clients to give POAs unless there is and absolute need and no alternative and only for specific purposes and of short duration? Especially to family members!!