I don't know what to say but if she is in danger of hurting herself she can not be released that is unethical they need to find a place for her, it is their job to protect her. I would suggest calling your states disability rights legal office they are there to protect the rights of everyone with disabilities, your sister needs treatment and they may be able to help, at least refer you to places that can help. Just look under www.disability rights(your state).org you can find the number to call.
Health care for the mentally ill is hard to find in USA, and if she is unwilling to be in treatment even harder. Best of luck. Please keep posting here. j