It sure sounds like someone messed up. In addition to being the beneficiary of her insurance policy, as the representative of her estate you maybe able to bring a wrongful death and/or medical malpractice cause of action.
Regarding the accidental death insurance carrier, they almost certainly have a provision in the policy requiring full cooperation before they would be responsible to pay on a claim. You should have an attorney review the policy. There is an extremely short statute of limitation on med/mal and wrongful death actions. If you sit on your rights, you may end up losing them.
The insurer is obviously trying to determine what specifically caused the death. If the proximate cause of death was by disease, or some other non-accidental cause, then they may get off the hook. However, even if your sister was terminally ill and had only a very short time to live, but she actually died as a result of the stated negligence/accident, then they might still have to pay the claim. It is extremely important to have an attorney review the policy to determine whether the death constituted an "accidental death" under the policy.
While you probably have to release the medical records, you should have an attorney review your obligations under the terms of the policy. Most such policies contain provisions requiring full cooperation in investigating a claim.
Who is playing lawyer by advising you that until you file a lawsuit you do have authority to request your sister's medical records? If you are the executor of her estate then you are entitled to request the records. Are you the executor? Has ther estate been probated?
You should schedule a formal consultation with a lawyer. A good means of obtaining an attorney referral is by contacting your State's bar association or your local county bar association. They probably have a web site and charge a nominal fee for the referral. Most attorneys either provide a free initial consultation, or charge a nominal fee. If you are located within the Greater Tampa/St. Petersburg area, I would be happy to meet with you.
Good Luck, Elliot Jay Goldstein (I also maintain a Tampa office)
The above information is provided as a courtesy, without any consideration and without knowledge of the specific facts. Do not rely on this information. Seek a formal consultation with an attorney.