This is a tough one, OK, and I sympathize and empathize.
I won't presume to speak to your situation, but I will share my thoughts about my own.
If my mother predeceases my addict sib, in whose house he lives, my other sib and I agree that we will make an effort to find him a nursing home, as that is about the only place he can go. He is too damaged by drink to function on his own.
We are not optimistic as he is an active drinker and I doubt any nursing home would take him.
We could dry him out and try to place him back in the sober house where he lived for a couple of years.
If he resists this, and he probably will, he is on his own.
he has made his choice. He has had multiple opportunities to turn his life around.
I don't believe that we owe family members caregiving rights, particularly when they have made detrimental health choices.
That being said, it sounds like the family member of which you speak has aging mental health issues, maybe some dementia, and is not going to make a good choices as a result.
Sometimes the only thing to do is to turn this over to a professional.
I would check the local elder care services to see if your parents can become their clients.
My mom became a client of our local elder care organization after a fall that landed her in the ER, and I cannot say enough good things about them.
I have been to her case manager several times with mom issues, and she has always been helpful and effective.
Good luck.