Currently in the same "family situation" and I completely understand your frustration. I also agree with those above.
They are doing this for themselves. It is something they need to do to fulfill a promise, to try to have normal memories, to hold on to hope that this will be a wonderful and joyous vacation. Maybe even that something may spark some light. It is very very difficult for some family members to accept that the person they knew has faded away and they can not accept who they are now.
But taking someone who has lost that cognitive ability out of their comfort place may spark unexpected anxiety, behaviors, fear and create a nightmare of a vacation. Traveling just to the doctor or store can be stressful, vacation travel can bring on all kinds of things. There is no comfort zone there. No way to get them home quickly. The point you describe their cognitive level at tells me that it is likely to be more stressful than positive. I agree if it is not harmful just because he does not understand but it could be if it could lead to some negative behavior, anxiety etc.
I have this exact family dynamic. MIL is finally in a home close to us. Half the siblings enjoy their time with her, cherish who she is now and do all they can to help her have a quality healthy peaceful life. The other siblings can not let go of who she was, can not relate to who she is, always pressing her to remember or say things she can't and ultimately stress her. We had a similar scenario - huge family reunion in the midwest. Plane ride. FIL wanted her to go, could not accept that we could take this trip and her not go. All staff at her home said it was a bad idea, most the kids felt it was a bad idea. He was determined. Sadly he suddenly passed away before the reunion. MIL was safe and we were still able to attend the reunion - in their case, do they care for him at home? does it mean if he doesn't go they can't go?
Questions:
- Are they flying? Is someone his legal guardian? If they try to get him on a flight without that authority and the security determines his condition to be severe, they may not let him travel. Who will stay behind?
- Is one of them going to always be prepared to stay in the hotel that day if they can not manage to go to the park?
- Are they continent and is someone equipped to deal with this? Something we have to consider since only the girls in family change MIL.
- Do they have an emergency medication to give them should there be a physical panic attack that they need to manage? This is not home. They could be in the middle of a park. I have been in middle of a park with DS with a full blown panic attack - it is very difficult.
I know as with our situation, you can't change some people's minds, and unless you also have some say so over this person's daily care, there isn't much you can do but hope that they remain safe. You expressed your concern, now I guess just hope all goes well. Again I understand what they want to do for themselves but sometimes truly loving someone is doing what is right for them. I wish them luck!