At her age, you would have to file a petition with the court that she is incapable of safely caring for herself. If you can prove that (you have pretty good evidence), the state would appoint a guardian who would be able to act legally on her behalf. She also could be court order into whatever facilitiy is most appropriate. They would be require to place her in least restrictive safe environment. This could range from assisted living to state hospital.
On a practical level, if you become guardian, the bills are yours to pay and likely 50-100 thousand a year, depending on placement, so think carefully before deciding to be guardian. Assisted living may not accept her due to fire risk, so treatment may be expensive
If state becomes guardian, they are responsible for cost of placement. They will try to get you of family to be guardian, but if you refuse, they will be responsible (rules likely vary by state).
You really need to consult attorney.
Guardian is also called conservator, just in case the term is confusing.
Please don't let her come live with you. This alone could ruin any chance of getting a proper conservator in court as you would already be voluntarily doing it. In some states/countries, once you take her in, it is almost impossible to get her out.
If needed, call 911 if she unable to care for self, and hopefully they will assess her and start the process.
She does need to be in dangerous situation before 911 is appropriate, so you may have to wait for a crisis.
My guess is a secure skilled nursing facility may be best, XXXXX XXXXX will likely need the court procedure to get her in one.
The fire setting, albeit unintentional, makes her high risk and hard to place. Plus she will not want to go and lay on the guilt.
Yeah I really don't want to see her end up in a state hospital - but some sort of assisted living thing would be great. I don't have any money to pay for any such thing though - can't they take it out of her SS and medicare?
You know what? I'm greatful for the response. I'm going to repost all of this over on the legal justanswer portion. Thank you!