Well, normally the hospitals do give information to relatives of a patient if he/she calls them up. This is not against any hospital's policy. So I would day you can get information about the patient if you pose as his/her sister.

 Because of  prior problems, hospitals now put new rules into effect about patient information, so this would depend on the facility itself.   Since you said if you say you are the patients sister. Are you really?  What if the patient was there  because of  someone harming her.  They try to protect the patient as much as well as possible. If you are a good friend, just call the relatives of the person, and if you can visit, do so. Many hospitals when they admit a patient, asks the patient for names of the people who may inquire about his condition.  If you call and other rules are in place probably all you will get is:  He had an uneventful night, he is in therapy, or taking tests or just unchanged." If the patient has been dismissed or expired, they may simply say they have no information on having a patient by that name.