I know this sounds harsh, but the only way I am able to see this is with the thought in the back of my mind: Time to grow up. You two brothers sound like a traditional couple, co-dependency issues all over. You feel responsible for living up to the expectations of your brother, your brother feeling responsible for bringing you up living up to the expectations of his own moral standards (or the standards he regards as the ultimate ones). Does your brother even have some kind of own live he is able to influence? He seems to live solely through you and your life.
Just for clarification (I hope I didn't overread it somewhere) but your brother is mentally healthy in general or are you in charge of him and responsible to look after him on a daily basis? Because the way you describe him sounds as if he would really need someone to help him lead an own life finally. But that person should be a professional, not a loving family member without any training or qualification in that regard and most importantly: someone without personal involvement. So if he comes back from that hospital, I would inform me thoroughly (if possible) what the problems behind his reaction were. If he is scared that you may leave him, that he looses his connection to you or if he was just unable to gauge the 'moral-dilemma' of you, a dear person, doing something so utterly wrong to his mind.
And don't give him this much power over you. You have the right to live your life the way you see fit. His constant need for care is highly controlling, because you have to consider him every day of your life and it influences how you yourself regard your decisions if I got that right. Look for professional help. He did so himself, so he is either able to see the threat he holds to his existence right now or he wants to get your attention by this. Maybe both. Don't get further succed into this powerplay and don't handle things outside of your capabilities.
This would be what I would try to do. Sorry if I am way off, because I personally never dealt with this kind of person. So take this with a grain of salt, I may be wrong.
Facts: 30, female, bi, v-type relationship with Sward (husband, straight, mono) and Lin (boyfriend, straight, mono), poly-fi and co-primary.