How blunt have you been?
I wouldn't spare their feelings at this point.
I would spell it out in black & white, if you haven't already, and tell them that when they do X, brother does Y, and the you feel it's cruel for him to be put through that kind of pain.
Since they don't put any credibility into the threats, I wouldn't even go there & I would focus on what they should care about.
But, maybe they'll never accept how severe his illness is. My husband made every excuse in the book for why our son did not have a severe, probably lifelong, mental illness. And, some days, I still don't think he gets it.
Maybe you should also talk to them about what will happen to him when they are gone. And, if you haven't read it yet, I'd get a copy of I'm not sick, I don't need help by Dr Xaviar Amador. It teaches a way to repair relationships with your MI loved one and hopefully convince them to get treatment even if they never have any insight. I'm still working on the methods they suggest, but I see how it can help with communication in all kinds of situations outside of mental health. It could even give you some pointers on how to talk to your in-laws since they kind of lack insight too.
I think I'm not the only one who thought that because he also wrote a slightly different version that I bought too called I'm right, You're wrong, Now What? That's kind of how I approach everything in life, but it does not work with my son, so since I have very limited power to change him, I'm working on changing myself.