My sister, brother and I share POA for our mom. Can my sister legally have the facility withhold records if I am on the POA?

My mom, who has dementia, has my sister, my brother and I as her POA. My sister is first, then me, then my brother. When my mother first moved into a memory care unit we used the Physicians House Calls service where a Physician Assistant was seeing her. Initially my sister did not see the need for her to attend moms appointments and when I highly suggested one of us be there to be moms spokesman she give me Physicians House Calls information so I could attend any appointments. Using this service meant there was not a set appointment time only the day mom was to be seen so a person could be there several hours waiting for the physician to see my mom. Plus I live over an hour away so this was a full day for me but worth it to make sure mom had someone to help her during the visit. Mom had been swelling in her knees so had the PA look at her swelling. He put her on Lasix and they took blood and an x-ray. But I noticed the PA asked questions that a dementia patient could not answer plus they had her age wrong. Over the next few months mom kept putting on weight and her swelling increased as well as her pain level and mobility was getting worse. I notified the head nurse she needed to be seen again but they just pushed this off as arthritis and gave her Tylenol throughout the day. In the meantime I got the name of a very good doctor that I felt would give much better care plus easier for me with a set appointment. My sister finally gave the ok at the beginning of the year for me to make an appointment with the doctor. I took my mom to this doctor who said she wanted to rule out any cardiac or kidney issues that could also be causing the swelling. Mom had blood work done which showed possible heart failure. The doctor referred mom to a cardiologist who had a couple tests done to confirm that indeed her valves in her legs were not working correctly and causing fluid retention. The cardiologist gave us a couple of options to help mom but ultimately left the decision to us. We decided to monitor mom and made a 3 month follow up appointment if her swelling did not get better. During this time my moms knees were causing her a lot of pain and the Tylenol was not working. I took mom back to see her doctor about pain management. She was prescribed a mild anti-inflammatory and her BP med was also changed due to a cough she was struggling with that could be caused from her BP med. A few days later my sister called and said she got a call from the memory care that mom was having some possible blood in her urine/stool. My sister thought it was from the new meds but ultimately the cause was never determined. My sister was not happy and took her out of the care of the physician and back to Physicians House Calls under the care of the PA. A week before mom was to go to her return cardiac appointment she cancelled the appointment because the PA stated that the cardiologist just wants the money and that he (the PA) has worked with other cardiologists and knew how to take care of moms heart issue. My sister told the cardiac office that mom was doing better and did not need to be seen, which was not true. My moms swelling was getting so bad that my sister got bigger shoes due to shoes too tight! When her and I spoke about moms past medical history and why she cancelled the cardiac appointment we had some discrepancies so I decided to get the facts through medical records from the doctor and cardiologist. I tried to get them from Physician House Calls but they would not give to me. I was very blessed to get in to see the cardiologist just to see what options there were for mom but he wanted to see mom so they opened a spot 2 days after the cancelled appointment. Physicians House Calls called my sister and brother and they showed up to the appointment. I was glad so we could all hear what the cardiologist had to say. He put mom on a good plan to get rid of the fluid, had bloodwork done and wanted to see mom in a week for more bloodwork. Way better results then when the PA was seeing mom. I have tried to pursue getting medical records from Phy. Housecalls and found out today that my sister told them not to talk to me. Can they legally withhold records if I am on the POA? I am also very concerned about a PA giving us advice to not have my mom see her cardiologist for her cardiology needs. Is he violating his scope of practice and if he is what can be done? Not sure why my sister has "banned" me from speaking to the PA office. I know I will have to sit down with her and find out more of her reasoning. Any thoughts??