I am having the same problem with my mother in law right now. It is horrible, and I feel like everywhere I turn the rest of the world either doesn't understand or is too stupid or busy to care. I don't know where you live and the laws are different everywhere, but in Texas you can have her committed to a Psychiatric facility for up to 72 hours. Problem is, if she is a manipulative person like my MIL, then she can fool the pros if she is only observed short term. Then they have to let her go. If you can get other family members to help you, you can all get her declared incompetant and then put her in a facility. But they don't help much either. My experience has been that they are short of staff, don't take the time to document everything like they should, and fail to communicate with each other and family members. I wish I could help you more, but I am in the middle of this right now(lawyers from one relative are sending police out and making complaints, threatening court actions, revolking power of attorney, etc. ) We are so drained and sick, that we are considering letting her out and cutting all ties with her forever. She will probably go back to her little apartment where no one will help her and die shortly afterwards. We sound mean and cold, don't we?But that is what happens when you deal with a malignant narcissis with dementia. It is her survival or yours and your family. We have young children and they are suffering, too.
Keep me posted. I really care, although I don't know you, I feel your pain very much. You obviously love her and are trying to do what is right for her. The decisions you will have to make are hard. I suggest you keep a folder of everyone you talk to: Psychiatrists, mental health facilities, nurses, etc. Document EVERYTHING! It will back you up later in court if necessary, and it will help the doctors to make the correct diagnoses. Good Luck!