Is there anyway a POA can stop being revoked?

Yes, if you take your Mom to an elder law attorney and this lawyer interviews her for capacity, and she passes, then she can create a whole new PoA. Not sure what the process is regarding revoking, this may differ by state. A good attorney will make sure that your Mom is not feeling pressured to make this change (which they do in the private interview). If she's able to get this done then she (or her new DPoA) need to go to every institution to present the new PoA documents and make sure the sister no long has access. Not sure how this will happen at the bank if she is joint on an account with her. It varies by bank.

If you have hard evidence that your sister is abusing your Mom financially, are you going to report her? Talk to the attorney about this as well.

Your mother can do it, she really doesn't need a copy of the old one to move forward. In this case an attorney would provide the help you need.

I never will understand why people don't keep copies of important documents, baffles me. I always have two originals of everything signed, one for me, one for the other person.

Your mom has to do it.

Yes, she can revoke POA anytime. Speak to a lawyer or notary to see how to do it.

If she’s able to sign, it means she’s competent. This means she HAS access to her accounts right now. She can go to her bank and get access. She can tell the bank, the daughter is no longer allowed to have access. The bank will revoke access right away.