Unfortunately they’re both terrible to each other. Always have been, likely always will be. Even in relatively stressless situations they end up at each other’s throats in seconds.
The family (not my wife) asked me to talk to my brother-in-law about this because nobody else could get through to him.
I totally agree I’m not the right person to get in the middle. There’s just nobody else willing or able to.
If this is the situation, I would not be so hard on yourself. It seems like there's a lot of history and emotional trauma that would need to be resolved before any progress would be feasible. If this is indeed the situation, getting your wife involved in the process could make it worse, not better. Imagine all the fights they'd have with so much on the line, when they'd have so many fights before when there wasn't much on the line. You just walked into an impossible situation and did your best. You probably would have done better if you were a sympathetic complete stranger with whom your brother-in-law had zero emotional history (and even if he had zero emotional history with you, he certainly had a lot with your wife and probably subconsciously viewed you as her proxy).
edit: Thinking about it, I think the only thing I'd be able to say in that situation is something like this, just be supporting, not judgmental, not trying to convince him about anything: "Hey man. I'm really sorry this is happening. I know that things are hard right now. If there's anything I can do to help you or the situation, let me know, and I'll do it. How are you yourself? Do you need to talk with anyone about how you're feeling? Let me know if I can help, I'll be right here, OK?"
> The family (not my wife) asked me to talk to my brother-in-law about this because nobody else could get through to him.
I am a stranger on the net but your first priority is your wife and your relationship with her. Be very cautious with demands from the family even if well meaning. Protect yourself and your relationship with your wife.
I second this comment. If the wife has not asked for the intervention, the first obvious question is, why not? That's something that needs to be cleared up with the wife before anything else is done.
When going through a similar event, I was able to find resources at the hospital that helped a lot. In life and death situations they often have patient advocates on staff. I was able to reach out and had a patient advocate, either a nurse practitioner or doctor, that helped sort out the communications. A lot of the stress came from doctors' reluctance to put things in simple terms. The patient advocate was able to basically come in and say, "Look, they can put a pacemaker in, but your loved one only has 12% heart function and is still dying." It was received differently than a family member saying it. After all, their job is to advocate for the best interest of the patient. Ours actually got pretty angry with the doctors for how things had been allowed to progress. Hospitals usually have grief counselors and social workers as well. Just getting the brother extra support might be enough to open him up.
If this is the background, then your failure with your brother in law was probably inevitable, but trying was important for maintaining and improving your relationship with the rest of the family.
If you had just said, "forget it, it won't work," and not tried, the outcome for your wife would be the same, but everyone else would also be mad that you didn't try.
> The family (not my wife) asked me to talk to my brother-in-law about this because nobody else could get through to him.
This is probably a central part of the problem: you're the worst person to have the conversation, and you were brought in because everyone else failed, so he was primed against the subject matter. You started in a hole you had no hope of digging out of.
> The family (not my wife) asked me to talk to my brother-in-law about this because nobody else could get through to him.
This is a very tough situation and I don't think it was any lack of skill on your part that made you unable to get through to your brother-in-law this time. I don't think the world's champion hostage negotiator could have done it on the first try.
I'm a little unclear on one aspect, though. You say your goal is to get your brother-in-law to allow your wife to talk to medical staff. But the family not your wife is asking you to talk to him? Yet they're not asking for anyone else, other than your wife, to be able to talk to medical staff? If there are aunts and uncles, presumably some of them are siblings of your father-in-law, as closely related as his children and with as much right to be involved in medical discussions as your wife has. Yet they're not trying to get themselves involved in the process? Only your wife?
(Also, as I commented elsewhere in the thread, if I were in your position, I would not want to take this on unless I was sure my wife was OK with it. I can totally see that she might just not want to have the discussion herself because of her history with her brother; I just would want to be sure that she was OK with what was being done.)
If there are aunts and uncles, presumably some of them are siblings of your father-in-law, as closely related as his children...
A technicality: Not sure in the USA, but over here that's not correct. Relation is measured in grades being those the basic parent-child "distance". So you're related in the first grade to your parents or children, but in the second grade to your siblings (1 grade distance from you to your parents, another one from your parents to them) so, unless the father in law has a living parent, no one is more closely related to him than his children.
So for lack of a better way of understanding, I would just ask a lawyer.
The family (not my wife) asked me to talk to my brother-in-law about this because nobody else could get through to him.
I totally agree I’m not the right person to get in the middle. There’s just nobody else willing or able to.