Can a person who has Dementia and who is narcissistic also be a hoarder?

Understand, that if a person enters a facility "for observation", the bills will not get paid by Medicare, most likely, unless she is enrolled in an HMO advantage plan.
A person who cannot take care of themselves, can be a very sad and frustrating situation.
It can be confusing, and every emotion imaginable.
They will, at some point, need to be taken into custodial care.

You, or neighbors or friends, can call 911 to get "welfare checks" done, even weekly.
When you call to request a check, let them know the conditions that exist in the home she is in, and what she has stated to you:
---very depressed statements, and, might be considered suicidal.
---person who is a "danger to themselves or others",
can be taken in for evaluation, which leads to other care, if needed.

Officers sent to check on her, will find what you find:
--house full of trash, spoiled food, unclean, infestations, fire hazards, etc
and document that.
IF she acts out while they are there, or refuses entry, that also gets reported.

These visits and statements are then on record,
and can lead into having her put into protective custody for evaluation, then placed in an assisted living or nursing home, or an Alzheimers unit.

It is a tough situation.
Best thing is get her placed into a facility,
since taking her into your home would end up with your house looking like that, too.
IT sounds like she has some pretty deep depression going on though, and that she might be a danger to herself or others--if not directly, then indirectly, such as if she lights a fire, and burns the place down and injures herself or others in the process.

Please take care!
Please keep us posted!

Yes, they can have an array of problems! They don't discriminate!! ;) Sorry, my warped humor! Yes, there are often multiple problems under the surface the appear as the mind goes.

Yes. If you go to a judge you can explain her behavior and ask that she be taken to a mental facility where they can evaluate her. They can evaluate her and recommend that for continuity of care she be admitted to a nursing home. This is one way to have her put in a place for her safety against her will. You can also call Adult Protective Services and ask for their help. Just explain the circumstances to them they can also help.

OH, YES, to your question. If she is as bad as she sounds she may not be capable of making sound decisions You could pull rank and get her into a long term care facility where she will be supervised. At some point she doesn't have to 'agree'. Are there other siblings who disagree that she's that bad? I'd meet with a facility to start the process of 'how' and 'when' and also an elder law attorney about her home and finances. Take care and brace yourself. You are trying to help her.