I am a law school graduate currently awaiting Bar results. What I offer to you is mere information, not to be construed as forming an attorney-client relationship.
Heidi,
I answered one of your questions in your first posting. Did you read that.
Here, if there is more than one defendant, the guy who ambushed him and wanted to beat your brother up is going to argue he was not involved in a conspiracy or had no knowledge that the other guys were going to shoot him.
However, he may be an accomplice or prinicple depending on how the prosecution will paint the picture. Possibly saying he knew, he encourage them to shoot, he had knowledge or reasonable knowledge they would shoot, etc.
as far as rights of the parties, that is a very general question, they have a right to a defense and are innocent until proven guilty. If the prosecution chooses to call you as a character witness for your brother, you have a right to speak and be cross examined by the defense. If they are found guilty, you have a right to speak at the sentencing, offering victim impact statements.
Hope this helps.