If there was a will then the person named in the will has to have the will probated and have himself or herself named executor (personal administrator). If there is no will the person listed as next of kin in the state's intestacy law may be the appropriate person to have appointed as the administrator (analogous to an executor, but serving under state law as there is no will). Caution - the terminology differs by state.

There may be a simpler and cheaper option - if you were designated agent under the sister's health care proxy that document may give you access to medical records (depending on the terms of the document).

If you're looking for records to prove a malpractice case hire a lawyer as soon as possible becuase there may be strict statutory periods in which you have to file to preserve your action.