Bipolardude...you, and so have alot of others here, have surely wronged many people in our lives. I know with myself, the guilt did sometimes eat at me to where I felt the need to explain myself to people. But what you must tell yourself is to only worry about the meaningful relationships. There is no need to go and revisit every person you feel you have wronged. Depending on the time that has passed, most people have simply moved on. If life somehow brings any of these people back into your life, maybe you will get the opportunity to tell them "by the way....."
But as far as your brother, if it is a relationship that you care to reconnect, then you should go for it. He will always be your brother, and the two of you share many memories. You might be of help for eachother to get over the abuses you suffered. But you must go into it without too high of expectations. If he has truly cut his family out, he might be resistant to you tying to get back in. You know, he may need to see to believe.
I wish you luck, and think it would be a good idea to get back in touch and try to mend things.