First make sure that your sister has the powers she claims to have. Make sure the power of attorney document entitles her to act at once, before your father is
If she truly does hold the broader power to act at once, there are still some legal rules and regulations that control what she can and cannot do with your father's money and other property. Specifically, she must:
unable to act on his own.
- Act in his best interests
- Keep accurate records
- Keep your father's property separate from her own, and
- Avoid conflicts of interest, such as using his money to float her own credit card payments.
Your sister must be able to verify to your father that she is performing all these duties -- and you and the other siblings can have access to this information if he allows it.
You can try informing her that these responsibilities are included in the powers that come with a power of attorney. In a surprising number of cases, even the biggest bullies and diehard control freaks will change their evil ways and comply.
If this type of informative intervention doesn't work, you may have no recourse but to make the waves your father wants to avoid by filing a lawsuit challenging her fitness to act as his agent.