This is not necessarily personality disorder
IT can be depression.
It can be bi polar.
She needs to see a psychiatrist right away.
To approach this, I would mention that the family is very concerned and that she can not really give a medical evaluation of herself.
This could even be hypothyrodism, which when severe, has psychological effects.
She could thus start with her regular doctor, who can do testing and refer her as needed.
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In fact, if it is determined she is a danger to herself or others, she can be admitted and assessed in the hospital involuntarily.
Short of that she cannot.
I suggest that you are at the point of a decision, and that you should seek guidance yourselves, in person, from a family therapist who has worked with families in this situation.
A referral from your GP would be useful.
Then you can come to terms with any decision you might need to make.
This will take work on your part, and therapy for your family.
Currently, you are unable to make necessary decisions
For example, live with it, have her committed, make her live on her own if she thinks there is nothing wrong with her.
Any of these might change her situation.
Or, even starting therapy yourselves and inviting her until she comes with you, and might listen to the therapist.
Those are what I see as the options.
These are recommendations we make daily in medicine.