You have been misinformed. If your sister is seriously ill, she can receive treatment. The mental health act was created to help in situations where a person's illness prevents them from realizing they are ill. The criteria for involuntary treatment include risks to health from self-neglect or severe worsening in health from lack of treatment. Here's a link to the guide to the mental health act: http://www.bcss.org/2008/01/resources/f ... ealth-act/
People with schizophrenia need medication to get and stay well. It's not good for their brain to be actively ill, so the faster she get's medical attention the better. If she doesn't have schizophrenia, her symptoms could be caused by anything from a brain tumour or infection or other serious problem, so a doctor needs to sort out what's wrong asap.
In practice, it can be hard to get access to treatment, as mental health services and beds for critical patients are underfunded. However, it's possible. I suggest contacting your local BC Schizophrenia Society contact for some help figuring out how to go forward. Here's a link to the listing of contact information: www.bcss.org/category/regions/
Here's some information on how to advocate for your sister with the medical system. http://www.bcss.org/2008/07/resources/h ... loved-one/
Psychosis (paranoia, hallucinations, delusions) can be triggered by extremes of stress, hormonal changes (after giving birth, menopause etc...), drug use. If you have people in your family who had a mental illness, or who committed suicide or had a serious alcohol or drug issue, it's possible that brain vulnerabilities run in the family. This history might be helpful to the doctor that examines your sister.