Sweeney was the best thing about American Gods, oh my gosh. I couldn't have asked for a better portrayal. The faux hawk is a necessity!
He and his brother were close as kids, outdoorsy rough and tumble sorts. The did the cops and robbers, cowboys and indians games for days on end. It was probably Starks Vs. Lannisters, Starks being the bad guys. Hah. Specifics of the injury I'd nail down with his player, but when they were young and starting their training, Aeron shot him in the leg. It could have been a lot worse, but it never healed correctly. If his brother had any aspirations of a military career, it was over then. He carries a lot of guilt about it, even if they probably dodge the issue for the most part. His brother is darker, a bit more grim, and far cleverer than Aeron. I'd guess he probably fully invested himself in other arenas to take his mind off of the disadvantage dealt to him. The rest of his family is golden and lovely and always settling scores.