Perhaps you're assuming that this was a two man operation. I highly doubt it. Too many unexploded bombs were found in the vicinity for two guys to have planted them all.
Then there's the mysterious case of the Saudi (Al Harbi?) who was injured in the blast, but is now suddenly on a fast track to deportation. Lots of other loose ends have popped up as well, and are quickly being tied together as we speak.
Point is, these youngsters didn't commit this atrocity all on their own. Terrorists never do. There's always a hidden command and control network behind them.
Those are good points especially about more than those two. I was only comparing the two brothers. Older one was a 5-prayers-a-day Muslim boxer with stated ambition to be in the Olympics. The younger went to college mostly to socialize. There are many cases where older brothers led younger brothers in criminality (I’m not saying the opposite is impossible, just less likely)