Stepmom has Alzheimer’s and her sisters had her sign POA to them knowing that she’s unable to understand. What can I do to revoke their POA?

Your Stepmom is out of money and property? Then the one thing we DO know is that they aren't after her money.
You can certainly file for conservatorship or guardianship, but are you absolutely certain you wish to try to do this? If family fights you it is likely the conservatorship will be given to her direct relatives, esp since she DID sign over the POA to her sisters. It isn't necessary for an elder to be completely without any dementia to give POA. If this was done by a Lawyer he will have examined if she understands what she is doing. My brother already had a diagnosis of probable early Lewy's dementia at the time we did the POA and Trustee of Trust with his attorney; he did understand he was asking me to act for him in all ways, and his lawyer made certain of that.
I would examine why you really want to take this on, take it from her family. Then if you want to do so you can file for guardianship; an attorney will be provided to your step Mom, and her relatives with POA will be allowed to testify and the judge will make a decision. In a fought case this can be VERY expensive. See a lawyer to discuss. Good luck. I admire your loyalty to your stepmom.

You need to contact an attorney. How do you know she's not competent to sign a POA document? If her mental condition has been documented in her medical records, you may have a case.