It depends on how much of a beating you gave him. Jokes aside, fights between loved ones can happen because we love them and we are easily hurt by them. Verbal arguments can hurt someone and become mortal sin if the honor due to the person is taken away (ex. if it is not just a loud argument, but it has grave personal insults or remarks that undignify the person). Same with good friends… Now, if the fight became more than mere words, I would argue that the deliberate consent became blurry…a Christian in his right mind would never lay hands on a brother or a loved one during a fight, so it must have been really bad…in any case, that definitely requires to repair and be reconciled with the person hurt, and to confess.
I would personally don’t risk it. I know about this sin and I know that if there is ever a heated argument, I am partially to blame (or I could say it is my fault, as I can always just smile and refuse to argue and back away, but I chose not to). I was once told, about arguments that involve critical things, that I am to be firm but kind. Easier said than done, but it is the only way when backing away is not an option.
Also age is important. Two kid brothers fighting is probably not a bit as bad as two adult brothers fighting!
To understand how common an occurrence in close communities fights are, suffices to say that the ancient Rule of St. Benedict has one section called “That No One Presume to Strike Another”

Then again, to avoid occasions of arguments another section states “No One Presume to Defend Another”. I think it is a worthy reading.