A legal guardianship or conservatorship of an incapacitated adult generally ends when the person for whom it was created dies. Your question seems to imply that you don't feel your
The laws about the privacy of medical records are strict. The intent behind that was a good one: Well-meaning legislators wanted to ensure that personal medical records did not fall into the wrong hands and that the information in them cannot be used to discriminate against a person. However, as you’re finding out, ensuring privacy can mean that it's awfully tough for outsiders to get to the bottom of the situation when they suspect something has gone wrong medically.
And many doctors and hospitals are also notoriously close-lipped when it comes to explaining the whys and wherefores of medical procedures, even to involved family members.
If your mother was hospitalized, your best bet may be to contact the hospital ombudsman or patient representative for help. If you suspect wrong at the hands of someone in a health care organization, consider contacting The Joint Commission, which should oversee such complaints.
brother did the best job as your mother's guardian. You should be able to check the reality of this, as most state laws also require a guardian to file periodic accountings with the probate court. "Interested people" are also generally entitled to see these accountings, and as one of your mother's likely heirs, you would probably qualify.