84 year old Parent gave up home, personal belongings, changed church denomination, to please new spouse. He wants to move to a community where he can golf, dine and have a social life that includes new spouse. She would have to leave her house of 40 years, and refuses to even consider a move, even after Dad has obliged all her requests.
Are there other indications that something might be not quite right in this regard? I suppose you've looked up typical dementia symptoms. Are either of them showing behaviors on that list?
This married couple apparently disagrees about the best place to live. Your dad is giving in to his wife's wishes. I don't think that in itself is a sign of dementia, any more than if his wife agreed to move it would mean she has dementia. They both can't have what they want. This is a conflict, and it may or may not be an ongoing source of tension for them, but I don't see it as a reason for family to intervene. Sure, give your opinion if you want to, but avoid the risk of alienating them. A continued good relationship between you and Dad is more worthwhile than trying to get Dad to stand up for himself.
"Dad is giving up every part of his personal life to have her companionship." I can understand why this upsets you, but, really, is it any of your business? Looking over the marriages of my children and my siblings and even close friends, it is easy for me to see how the relationship would be "better" if they tweaked certain elements, mostly to be more like what I consider an ideal relationship for me. But it is certainly not my place to tell them that! (Nor would my "reasoning" with any of them likely to have any impact on their mariage.)
It's not unusual for one spouse to give more than another. You dad has decided that her companionship is more important to him than other things, even though the other things are important to him also. He has made a choice.
If you feel that his new wife is hurtful and unkind to him, then I am sorry to hear that. If the demands that she has are not beneficial to his health and security, again, I am sad to hear that and can understand your distress.
As to each of their cognitive abilities, if she is suffering some decline that may be all the more reason she does not want to make changes in her living routine. If your dad is having some cognitive decline, then maybe he feels more dependent on her and feels safer in her company.
Naturally, these are all guesses on my part because I don't know the circumstances other than from a couple of paragraphs you posted.
If your frustration comes from his wife not giving her fair share, then I think you are asking for trouble. If you are concerned about your dad's health and his wife's health, maybe you can get more involved socially with dad and spouse and see what you can do to make their lives more enjoyable and easier.
It's hard when elderly parents make decisions that worry us, but if they are of sound mind there is nothing you can do about it. If you want to discuss your concerns with your sibs, that's fine, but be careful that the actions you take do not cause your dad a lot of unnecessary pain and unhappiness.
Take a gentle approach to this and be there to support him and love him. It will get you further in the end.
Cattails
However, I will advise you to sit down and have a frank and open discussion with your dad about his finances, his will, and his wishes. As a married couple, they are free to do as they please. But, you will surely be plunged right into the thick of things if she dies before he does and he's left penniless or homeless (you said it's her house) or if either of them dies and there are questions about the co-mingling of their assets. That's where I am now. Dad's virtually penniless (having spent all of his meager assets getting the wife out of debt) and stepmom is resting in peace. Her kids said thanks to my dad for nursing her through her final, grueling, year-long illness, booted him out of 'her' house, and walked away. His up to his eyeballs in debt and it's all in my lap. There's a cautionary tale for you.
Sounds like these two adults are having some conflicts in their marriage. Presumably they'll work them out ... or not.
Why are you taking a role here? You want to "reason" with your Dad to get him to "part from spouse" -- have I understood that correctly? You think he'd be better off leaving his new wife so that he can be closer to a golf course? And this is your business because ... ??
There must be more to the situation than your brief paragraph explains. Maybe if you provide more details someone will be able to give you some suggestions.