I don't know if this will help or not, but you can also be involuntarily committed in my state if you're considered gravely disabled, which means unable to provide food, shelter, and/or adequate clothing for yourself due to an acute mental illness. Even if you deny suicidal/homicidal ideation, you can be committed if they find you to be gravely disabled. I have schizoaffective disorder and denied suicidal and homicidal ideation to the ER psychiatrist when I was taken there, but because the ER pdoc felt I was gravely disabled based on the info he got from my friends/primary care doctor, I was involuntarily committed. It wasn't exactly the best experience ever, but when it was all said and done, I was relieved I'd gotten the help, and my friendships are still intact--maybe even stronger than before, actually. I would think that if your sister was kicked out of her previous housing, that would count toward gravely disabled. I'm in Pennsylvania, by the way--not sure how things work in other states. I really hope this helps, and I am so sorry you're in the situation you're in!
Best Wishes,
Hope