Sorry to hear about your sister. What you are going through is something most of the families here have gone through at one time or another. You really want to get her treatment as quickly as possible - the sooner she gets treatment, the better the outcome (ie. chance that she will return to a healthy life). The best places to go are the Early Psychosis Evaluation and Treatment centers - see if you can find one close to you:
The whole issue of “forced treatment” is a really hard one - every family has to make their own choice. What would you want your sister to do if the situation was the reverse and you were the one becoming psychotic.
The best way is to get her to go to treatment voluntarily - and for that, what most people try to do is to get them to identify some issue that they do agree that they have (perhaps stress / anxiety, depression, etc.) - and then prewarn the psychiatrist about the true situation.
Write down a journal of all the unusual behaviors and thoughts that your sister is having - and document it so you can share it with the psychiatrist at some point. (obviously keep it far away from where your sister will ever see it).
Have your family watch these videos online:
and
and buy his book too if you can: