I'm having trouble with my younger (21y.o) brother behaving like an apathetic obnoxious tool...

My bipolar doesn't make sense in his logical mind, so it's not validated at all. My solution to that situation is to not talk about how I feel. BUT... He still gets to moan rant or gloat bout his life, relationships and whatever... So I just sit there and listen.but i feel it's UNFAIR! Why does he still get to share?? He invited me to go hiking, when yesterday came around I really didn't feel like it, so I graciously declined. His response was to guilt trip me for 45min straight, and when that didn't work, he switched to insulting me for 15min. How I managed to hold my tongue instead of bash his face in is beyond me, (yeah I got temper issues) but I'm proud of my lack of response. When he got home he tried to strike up a convo like nothing happened. If that wasn't awkward enough, he told me he feels hurt that I didn't enquire about the hike, I DON'T CARE ABOUT THE STUPID HIKE! thought I made that clear... Argh

Anyway thats just a drop in the ocean, his favourite response is "I don't care", or "that doesn't affect me", or "how does this affect me?" he's always been a douche but he's worse after our dad passed. He's mean, obnoxious, demanding with a filthy superiority complex. And incase I wasn't clear, ONLY his opinion counts.

I suck at finding balance, I'm black or white, all or nothing, problem is, I don't want it to be 'nothing' between us, but I'm struggling to draw the lines and set boundaries... I know he's an arrogant fool, but I'd like to have some semblance of a relationship with him, our other siblings are WAY older than him (I'm talking bout 15-20yrs older) so I know he doesn't really have relationships with them...

Any ideas on how I could possibly have a relationship with this dude without investing too much emotionally?