The Attorney-in-Fact named in a Power of Attorney document has an obligation to manage the business affairs of the Grantor in his or her best interest.
Typically, that means keeping personal, financial and account information confidential.
If siblings or other interested parties are unsure the Attorney-in-Fact is properly managing the affairs of the Grantor, they can ask the Grantor to assign co-POA to gain equal access, or they can petition the Court for the records they request.