Well... some bullets are worth biting. Because it's not really going to be FOR your BIL. It would really be for your husband's parents, especially his dad by the sound of it.

It sounds like John is an idiot and a waste of space. He's also not apologetic, and he doesn't deserve forgiveness from you in the broadest sense. But, you can bury the hatchet while still remaining fully aware of the level of lunacy you're dealing with.

This rift causes a pain for your in-laws, just knowing that their boys don't talk to each other. I don't think they're being pushy in voicing that they'd like them to get along. They seem to just being trying to have a nice family, something we all want.

Sometimes we just make concessions for the greater good. Screw John. Make it about your kids getting to know their cousins, having the whole family around at holidays, etc.

As your kids grow, and as your in-laws age, you'll must likely be glad, and if he does get the least bit weird you can say you tried!

Additionally, you don't have to be friends with the weirdo. Keep busy when he's around, don't engage, talk weather and baseball, smile and just remember he's fucking nuts!