My oldest sister was the poa for our mom. The day she told over mom's finances things got top secret. She wouldn’t let me or my other sister know anything about mom or her finances. Then we start noticing new thing at her house. To make a long story short. She just gives it up. Now we’re trying to get mom on Medicaid we’re going to need receipts. How would you do this? Need help.
Now, if your mom currently is not cognitively competent to create a new PoA, then either the family pursues guardianship through the court system, or the county comes in to eventually assume guardianship. No one receives Medicaid without there being a legal authority to manage their medical and financial affairs.
If it's not clear what your sister has done/is doing but she maintains she is no longer the PoA then you may have to consult with an elder law attorney for further guidance.
As POA your sister would/should have kept records to prove her purchases. If she did not keep these records, she may go to jail. Telling her this, after you get conservatorship over your Mom, may have her relinquishing her records pretty quickly if she kept them and is proud they show her innocent.
If your sister did perpetrate a fraud on her mother and steal from her your mother will need to (though her surrogate if she is deemed incompetent) file suit in order to still remain eligible for medicaid help.
Wishing you good luck. I would have a sitdown for sisters now. Time to discuss this before going to legal attorney help, but time to let her know you are willing to "go there" if you must. This may lead to cooperation and it may not, but at least you will know.
I would say that an hour of advice from an elder law attorney now will be very helpful. Take what evidence you have.