My sister is mom's POA,but I am full time caregiver, my mom has dementia, and I am home with her 23 hours a day, so I know my mother is not competent, that's why I'm here with her, my sister is refusing to take her for a competency test or any medical appointments, but she secretly takes her to sign legal docs and other important papers to get my mom's assets signed to her, what can I do?
That could be you or one could be appointed by the court.
To file a complaint you could contact the lawyer that handled the original paperwork and discuss it with the Attorney.
Or you could file a complaint of "Elder financial abuse" or possibly "withholding medical care" (if she is denying medical care) with APS and they will start an investigation.
Because of this is is far preferable that you and your sister work together. You have come to us with this issue in the past, and really I can't think of another way for you to go. Wishing you luck, and hoping you will update us when you have taken some action.
You can also speak with your Sister. You do have a bargaining chip in that you are currently the giving. Your telling the Sister that you will be moving out of State in a month and she will be in charge of all things involving your Mom may lead to some useful negotiating on a way forward for the three of you working together, rather than opposed to one another. Good luck.