Can I petition the courts to revoke POA of my mom with dementia for misconduct?

You can go to an Elder Law Attorney and explain what you have explained to us, taking with you what documentation you can get your hands on. You can ask to apply for guardianship of your mother, for whom you now care. The fact that you care for your mother will give you some standing. But, as I warn others, Hester, a family fight in court (your Mom gave POA to your sister and you care for your Mom) can, in some cases result in the guardianship going to NEITHER you or your sister, but rather given to the State so that an uninvolved paid fiduciary can assume guardianship of your mother. This could result in your Sister and yourself having no say at all in anything.
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.

If the POA is revoked your mom would then need a Guardian.
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.

You could try but having sister's POA revoked does not mean you can be assigned. That has to be done by Mom and she is not competent now.

How do you know that she's secretly taking her to sign legal documents to get her assets? If you're getting the info from your mom -- with dementia -- you know that info may not be accurate, correct? I'd make certain you know this is going on for an absolute fact before spending any time or money on it. What if you called APS and it turns out none of it was true...do you think it might affect your ability to provide her care? Just recommending that you are positive this is going on before acting on it. In the meantime you can video your mom (with a time stamp) demonstrating her memory loss or lack of comprehension. When you "petition the courts" what you're really doing is going after guardianship for her by you. If the courts smell an ugly family brawl they might deny guardianship to anyone and give it to the county. Or, you sister may remain as PoA. Pursuing it through the courts is very expensive since you will need a lawyer. Please consider all the downsides.

I would consider seeing a lawyer yourself! Or contact APS and tell them your concerns.