How do you deal with a family member who embezzles from your account or misappropriated funds, violating POA?

A number of years ago I granted a sibling POA for a select stock account to handle transactions for stocks in holding for my father who had also granted POA to this sibling for his financial matters at the time. Early on, the sibling borrowed money on margin from my account to buy stock for her personal investments. Additionally, the margin calls for unpaid charges resulted in the sale of my father's stocks to cover the lost margin money. I plan to engage a forensic accountant for a full stock evaluation of all accounts that can be identified for that time, including another $30,000 investment supposedly made in another sibling's name to hold Dad's assets. Those assets have yet to be located or reconciled by distribution or reinvestment.

How do I deal with the violator & with other siblings who don't see this as a problem & who defend the offender? I don't intend to file charges, but it's always an option even though my father has mandated it will not be done. I believe I have rights beyond my currently acting as his POA & trustee, rights that apply to the violation of my POA.

Your thoughts, besides only allowing POA to take action if you're incapacitated as determined by two medical doctors?