Bi-Polar is such a strange disorder, because it is exactly how it sounds - 2 disorders in one. The person goes through 2 extreme changes of personality.
Here is an article that might help
Is it Depression or BiPolar There are also different "levels" of BiPolar disorder. I suffer from what is called BiPolar 4 or "other" Bipolar disease. My manic phases do not appear like typical manic phases. I do not get extremely happy and excited during my manic phases nor do I go out and do lots of wild things. I get extremely iritable and angry. I will overeat, not sleep and overspend. Other people will engage in reckless sex, drugs, drinking, driving fast extreme sports, etc.
Then my depressive stage hits and I hit rock bottom - I cannot get out of bed and have been suicidal in the past. Now I know what this phase is and so can work through it without those dangerous tendencies - but it is still hard to leave the house and I am prone to panic attacks.
but if your sister truly is in the middle of a manic phase, then she will do everything she can to leave the hospital. She is seeking out reckless and dangerous behavior, because this is the only type of behavior that can give her the type of adrenaline high her body chemistry is seeking right now - at least until her doctors can figure out what medication she should be on.
There are many different medications used for mood stabilization of BiPolar. I am currently on one called Abilify. It is new for me and is working quite well. But some people do not do well at all on it - it makes them even more edgy. That is the problem with psychoactive drugs - doctors cannot predict what will or won't work for a person, it is simply trial and error.
While your sister might be able to fake some symptoms, it is doubtful she could fool an entire mental hospital. Your parents do need to educate themselves on where to draw the line between illness and when to hold her accountable, though. Part of getting better with mental illness is being able to be independant. If they baby her al the time - she will never have that.
The best thing you can do for you, your sister, and your family is to become informed.
Here is an entire website dedicated to information on the disease of Bipolar disorder:
BiPolar.com You will be in my thoughts and prayers.