You should just accept what you've done and take whatever sentence is dished out.
By the sounds of it, it'll be your brother getting the heavier sentence, so perhaps if you can prove that you understand your mistake, and regret it,then they won't come down too hard on you.
At least you won't make the same mistake again, and whatever the outcome, you can use it to turn your life around, do something constructive, and perhaps move somewhere else for a fresh start, away from violent thugs.