I am a law school graduate currently awaiting Bar results. What I offer is mere information, not to be construed as forming an attorney-client relationship.
It depends on what the defense atty is trying to argue. Does he have a choice to argue manslaughter or murder 2? If he does, then paitning your brother as an agressor can provide enough of a provocation to have the defense say "defendant was provoked" and thus did not "plan" the murder nor was it deliberate. This follows a similar line of fact patterns of the defense attorney's ability (allowed by judge or not) to argue self-defense.
But according to the rules of evidence, us lawyers have to abide by in court, the defense cannot bring in evidence of your brother's past criminal history to prove action in conformity therewith or to say that his past felonies showed he acted the same way this time, thus causing the defendant to shoot him. Also, the charges that were dismissed against your brother should not be brought up in court because they are not convictions. If they are brought up by defense, it is a dirty underhanded way for the jury to hear or at least try to taint the judge. A good prosecutor will object immediately and a good judge will "sustain" the objection...make it valid.
Hope this helps.
