There is a lot of published material dealing with difficult people, negotiating and compromising, and less than ideal extended family situations. When a family is is trying its best to work out an end-of-life arrangement for a surviving, and failing, parent, it is tough under the best of circumstances. It can be next to impossible when one family member is anti-social, narcissistic, and oppositional-defiant.
Some individuals want everything their way. There is no compromise; it's their way, or the highway. Those types of persons will sabotage the family's best intentions, they will seemingly agree, and then set about doing something different, they will find things to argue about when there is nothing to argue about. They will constantly make inane demands. These types of individuals will compare what they think they are getting, or think they should get, to that which others have gotten, or what they think others have gotten. They are never happy because they are miserable, unhappy people, and nothing makes them happy - NOTHING.
The sad thing is that the one that suffers from all of this thoughout the entire time dealing with it is the parent - the one everyone is trying to help. The parent bears the brunt of the adult child's inability to work in the family unit. The adult child, in essence, never grew up, and expects the failing, elderly parent to continue to parent them, step in on their behalf, defend them, support them, fight for them in the family unit. Unfortunately, the parent is no longer able to do so, and what ends up happening constitutes emotional elder abuse, if not physical and financial elder abuse, as well.
There are no easy answers, unfortunately. That is the inherent problem. The family has to deal with an unreasonable, irrational individual, usually one that has unfettered access to the failing parent. And, the failing parent, due to their failing, has little, or no defense to say enough, stop it, behave, as a parent might normally say. Going through the court system takes time, and is financially taxing, at the least. Meanwhile, the stress and strain continue.
Using a power of attorney for healthcare, and asking for assistance from healthcare providers may help, as well as contacting the county's elder abuse unit, to ask for assistance. Elder abuse, in any form, is against the law. Elder abuse usually occurs by someone the elder knows. Sad, but true.
When an elder is isolated from other family members, their email is censored, their phone conversations are eavesdropped, or cut short, or monitored, so that they are afraid to talk, that is elder abuse. When an elder is brainwashed by one family member that all the other family members are against them and the perpetrator, that is elder abuse. When everyone else in the family is wrong, and the perpetrator is right, about anything and everything, that is elder abuse.
There are plenty of good attorneys that specialize in elder law; find one and seek a consultation!