Hi Lost,
I'm sorry you've been carrying around this weight all these years. I have a feeling it might all be tied together.....being/staying in a disrespectful relationship, getting down on yourself for not having the power to change the past, etc. I hope you are able to find a support forum (maybe through Google?) and I hope you share it here when you do.
You did not do this. You did not cause your brother's condition. You did not cause your parent's behavior toward him. And the things you may have done that contributed to WHATEVER happened long ago were done by a child who didn't know any better.
You are going to have to forgive yourself some day.
You are going to have to come to a place where you realize that you did the best you could, given who you were -- and what you knew -- at the time.
I was never a therapist person, but I was able to work through a lot of this poison with a counselor, and stop carrying it around and letting it keep me in terrible situations, so guilt-ridden that I felt I deserved all the bad things that were happening to me. You might want to consider trying that, if you aren't already. You're carrying this 500 pound weight around, and it's keeping you sad......and you're not even supposed to be carrying it in the first place.
I didn't deserve any of it. YOU DO NOT DESERVE IT EITHER. You did the best you could with the light you had to see by at the time.
And you continue to do whatever you can, helping where you can, with your brother. Being there to support him in some small way, that's a good thing. You can't change his mental illness. You can't change the past.
You can only change now, today, and do the best you can for him, and for you. What are you going to do for yourself today?