Hi swan,
The fact that your sister is a nurse is irrelevant; patient confidentiality dictates that information cannot be given to a third party without the patient's consent. There are a few exceptions to this such as sharing information with other agencies involved in your care but even then only if there is a genuine need to know, also if the health and safety of others is at risk or if the law requires it such as in the case of notifiable infectious diseases.
Without your consent neither your sister or your parents are able to get any details about your condition or treatment. It is a disciplinary offence in the NHS to divulge information to a third party (except in the cases mentioned previously) without the patient's consent.
But to put your mind at rest I suggest that you make it quite clear to the person in charge of the unit that you do not want your family, your sister in particular to be given any information about you. Explain to them that she has a habit of trying to get information without your consent.
Don't worry about the two week assessment, just take it one day at a time and make the most of all that the unit has to offer.
Trudy