Since your father is unable to care for himself due to his dementia, he is not competent. Accordingly, what your sister did is attempt to make an incompetent person make a gift to her. It is wrong. If she won't voluntarily fix, and probably anyway, you need to apply to be appointed guardian to take over your Dad's financial affairs to care for him. Consult with a probate attorney who does guardianships.
There is no legal relationship created or implied by the exchange of message on this website. All statements are not to be construed as legal advice but as general guidance. In all cases, an attorney should be retained to review the full circumstances and deliver advice consistent with the information learned.
If the agent under the DPOA refuses to take action to protect the principal then you may have to sue her as well. You can file an application to be appointed the Guardian of the Person and Estate of your father. You will have to hire a lawyer.
DISCLAIMER: This is not specific legal advice and does not establish an attorney/client relationship.
Hire a lawyer and file to become your dad's guardian.
This answer DOES NOT establish an attorney-client relationship. This answer is based on the limited information provided and is not intended to be conclusive advice. There are likely other factors that might influence or change the advice after a more lengthy consultation.