How do you deal with aging parents who refuse to make their end of life wishes known?

Houndmother, good for you to try to avoid problems down the line. As a paralegal, you are well aware of the ramifications. I don't have a solution, only empathy. I have been involved for 19 months with legal issues, including 2 lawsuits, because my mother left a huge mess when she died and my sister hasn't fulfilled her duties as executrix of the will. It's too complicated to write but suffice it to say it has been horrendous. I've depleted the first of my retirement accounts and starting on the second one. The stress has been overwhelming. This is one of the saddest experiences of my life. I'm 63 and never imagined my retirement would start off like this. I had to get a lawyer because the laws are too complicated.
My husband and I have every thing in place and hope the 4 kids can come out of it unscathed. My experience is nothing I'd wish on them or even my worst enemy.
Good luck to you. I sincerely hope you can find some answers before it is too late.

Houndmother, my mother is on the much poorer end of the spectrum yet I've been going thru the same exact thing with her (no husband, tho). I just left her house yesterday after trying to approach the PoAttorney aspect with her as her dementia, etc is coming on faster and she's in bad health. She won't let go. She, too, sees me as meddling rather than desperately trying to help her. I'm walking away (she's in TX, I'm in CA), I know she's in good hands, tho I need to get her more at home care (see? Still trying, or meddling as she would say...). Her last atty told me just to let things run their course. If I was willing to step in when needed, so be it. If not, there was that too. I'm a loving daughter, and it's hard when, for some reason that WE don't understand, a parent perceives our intentions as other than what they are. But Veronica91 and Margot are so wise!

As of today, I hate this situation. As of "someday soon" I hope it's less painful.

I believe you and your siblings do have the right to appoint someone to see your Father , from a medical welfare standpoint. On the question of money , children have to understand they have NO right to dictate how a couple spend or deal with their money , if the wife is compos mentis. If you feel your Father is suffering abuse, physical or physiological, then you should be able to alert someone. Do you know any of their friends, this may be one way around the problem, your step mother is possibly feeling "under attack", you may personally have to step back from the whole situation, but write to your Father's doctor and just make him/her aware of your concerns.

What a dilemma!
As a paralegal you almost certainly know more about this than the rest of us.
You refer to your father's wife. Is she not your mother? does the wife have other children or any with your father? Do you have siblings? Have you discussed this with them. I am not sure that there is anything you can do to avoid the inevitable mess but there are things you can do to help your father. you said he has dementia. How bad is it? Can he still understand financial matters? What health problems does the wife have? Are you able to visit them? Is your father being properly cared for? Is the house clean? Maintenance up to date? Do you see any signs that the wife is diverting the family money? is the wife also taking care of herself/ hygiene? clean clothes/ cooking meals? If you do see anything bothersome you can of course call in adult protective services. Would you actually be the one who has to sort out the estate? they may have named someone else as the executor.
From my perspective as a 74 year old able to manage my husband's and my affairs although we both have significant health problems. I am not mean but I would be royally pissed off if one of our kids tried to meddle uninvited in our affairs. I wish we were worth 2 million but we are not but have done the necessary legal work so the kids have little trouble. That's my take, I am sure others who have been through this will contribute other ideas. Try and put this on the back boiler for the time being.

My dad's wife is not my mother, she is his second wife. I do have siblings, two younger ones who do not live here. I don't get the impression that the house is falling apart, or that there is current serious trouble with his wife providing care. Her main problem is back issues - she injured herself trying to prevent my father from falling - and mental issues. I don't know of any hygiene issues. I am able to visit them, but I live an hour away, and they no longer answer the phone or respond to emails. Whether or not my dad's wife is diverting family money, I do not know. I would think whatever funds they have would/should be set aside for his future care, as they do not have long-term care insurance. My father is not in a state of mind to manage any of their financial or day to day affairs.

I get the impression from your response that you feel I am meddling uninvited in their affairs. Please understand that I am not after their money, and am not plotting to take over. All I want is to see that my father is well taken care of, not abused, and is as happy and healthy as he can be in his later years. I do not trust his wife or her motives. I have witnessed her screaming at him, and have asked her to stop, which was met with yet more anger. Therefore I would not be surprised if there were abuse involved, however, I have no other evidence.

My motive is to prepare myself as best as possible for the inevitable, given that the two of them refuse to apprise me of their wishes or desires regarding any end of life issues whatsoever. I am trying to avoid dealing with a huge mess.

It may very well be that there is nothing I can do, except let things take their course and play out as they will. My offers of help have been continually rejected. Your advice to leave it alone is probably the best option. Thank you.