THe only way to "go around" the Agent is through a conservatorship/guardianship. The fact that the agent will not share information with you is not a basis for a conservatorship. If you think that niece is making harmful decisions regarding your aunts care, that is a different issue.
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Probably answer is you can't. Unless one of two things is present. If the POA was obtained by undue influence or while your aunt was not competent to execute a power of attorney. Both of these are very difficult to prove. The other would be that under HIPAA, if your mother is involved in your aunt's care, she can be provided information even without a written release. However, from what you described it doesn't seem this is the case either. The result is it may be very difficult to obtain information without the assistance of your aunt's daughter.
I agree with both of the prior responses that getting around the POA would be difficult. To be successful in obtaining guardianship to override the POA, you will have to prove that the POA isn't adequate and that the current POA has done something wrong.