I just saw this thread and honestly, I usually avoid threads like this because reading feels too much like work

I deal with families in crisis on almost a daily basis, and I can say you are not alone. In fact I have a FIL that must be related to your kin... they are exactly alike.
I just want to encourage you with one simple thought. I know it may seem simplistic, but sometimes the simplest observations are the only ones that make sense.
This is the simple truth... Maybe, nothing you ever do or ever say is going to make a difference in these people's lives. So often, when we deal with irrational, unreasonable people, there is just a part of us that gets so upset, because we want them to change. Everything would be fine if they could think reasonably. But I think you already understand that this expectation that they will "get it" is probably not going to happen. But here's the thing... I counsel people all the time with this simple truth... you Absolutely CAN control how their irrational behavior affects you.
It's OK to get mad, it's OK to be utterly frustrated (I would be), but it is also OK to set boundaries on how much of your energy and sanity you will devote to this. Give them clear caring instructions on how you plan on handling any further conversations concerning the matter... then just like with your kids, don't tell them something you aren't willing to do. Tell them you understand their desire and respect their opinion, but this is simply not up for any more negotiation or discussion, so please let it go. Then do just what you are doing... steer conversations away, then if they refuse, cut the conversations short. Let them know you won't be returning any calls or responding to any e-mails concerning this issue, then the minute you see info in an e-mail regarding this erase immediately, don't even read it.
Don't keep revisiting their irrational behavior and trying to fix it or help them "get it". They are adults (sort of), you are not responsible to train them.
Here's the thing, the irrational behavior of others is not the most destructive part for us all in situations like this. The most destructive thing for us is we keep revisiting the insanity, we want to change it, we want to explain it to them so they understand, we want them to "get it". Accept right now they aren't going to get it!! And choose not to give it anymore of your energy.
Will they keep trying to push your buttons? Yes. Will they just let it go? No. Irrational people are insistent that we give in to their demands, and they figure if they keep hammering long enough we will... don't let them have the hammer anymore

They may never let it go, but rest assured you can!
I will be praying for you... I definitely feel your pain
