Don't you mean: Do it with the character that would have an IC reason to do it? ![Wink ;)](https://forum.battlemaster.org/Smileys/default/wink.gif)
"Messing with my brother" is a perfectly good IC reason to do anything.
Family meant the world back then, so odds are all family members would have just as much to gain by hurting any said foe. In normal circumstances, that is.