Can other siblings question the financial POA's integrity regarding their parent's assests in regard to CD's?

Wow. The questions is not can you - plenty of people do. The question is should you. If you nose around on here, you will see two major schools of thought - one is that the sibling doing the caregiving is being unjustly attacked by the people who are not involved or contributing, and the other that openness is the best way to go, there really are "caregivers" who are all about robbing the caregivee blind and leaving nothing for anyone else in the long run, and if secrets are beimg kept, it is because someone is up to no good such that it is practically an obligation to find out what is going on.

SO - do you actually know who did the stealing from the safe? How do other siblings feel? Are you all pitching in to help either with respite or financially? Can you reasonably ask for a regular accounting to be sent around to everyone, just to keep the air clear and everything above board, or are your suspicions high enough and justified enough that you are going to need to consult your own attorney if that is denied?

Sorry more questions than answers. One possible answer if no one can see eye to eye would be something newer called elder mediation too.

A POA has no power after death. The Executor of the Will should be handling only your father's estate matters The POA of your mother should be handling your mother's affairs. You say "appointed"--by whom? If this was a Guardian appointment by the courts, the appointee answers only to the court that appointed them. Again, yours mother's POA or Guardian ends all control at her death. Again, the responsibility goes to the Executor of her Will.
Finally-- what are you doing poking around in Mom's safe? Sounds like illegal trespass, so be cautious.