Your sister has POA for your mom's money and assets. I think that settles it--unless there are indications of fraud, abuse, etc, I can't see why anyone else (daughter, son, etc) has any right to know anything about Mom's assets. Reference to an "estate" would give the impression that Mom has passed on--she hasn't. The money is hers, the person with the POA has been given the authority to manage it for her benefit.