I've never heard anything of the sort before, but hearing it for the first time makes me think that its possible. They are both loyal to Archaenia (lol spelling), and both wield an ultimate weapon. They both have similar colored hair too, but at first glance its difficult to tell if they look related or not.
They don't talk to each other in any of the games I've played, so my guess is that they probably aren't brothers. Most FE relations involve some sort of support/dialogue in the game, which is what is lacking here.