My brother, a physician who has DPOA of health care for my mother , tends to withhold some health information from the rest of us. For instance, after my mother, who is 92, had a laminectomy, he decided to put her in hospice because she isn't doing well but will not reveal the official diagnosis that gives her 6 months to live. I would like to gain access to the Hospice records to see what has been recorded as her specific diagnosis which is required to be eligible for hospice.
Based on what you said, my suggestion would be to get a big tough attorney involved immediately, so that the medications your Mom are given don't shorten her life span, too. Maybe start with a criminal complaint down at the police department and see what they say.... I can sure see why you are concerned. DPOA or not, long death process or not, we are not allowed to intentionally over medicate someone to death. Mercy.
Health care providers have flexibility when working with people who are helping a family member with health care arrangements. A government website answers Frequently Asked Questions and explains:
"The HIPAA Privacy Rule at 45 CFR 164.510(b) specifically permits covered entities to share information that is directly relevant to the involvement of a spouse, family members, friends, or other persons identified by a patient, in the patient’s care or payment for health care."
http://www.hhs.gov/hipaa/for-professionals/faq/488/does-hipaa-permit-a-doctor-to-discuss-a-patients-health-status-with-the-patients-family-and-friends/index.html
If your brother has given intrusions to the health care providers that makes them unwilling to talk with you, talk to an elder law attorney for ideas on how to help him cooperate with you, other family members and your mother's health care providers.
If your mother is able to communicate verbally, the regulation mentioned in the FAQ explains that your mother can verbally give permission to the health care provider to disclose her information to you.
https://www.gpo.gov/fdsys/pkg/CFR-2003-title45-vol1/xml/CFR-2003-title45-vol1-sec164-510.xml