I think this is the right call. The only thing that I don't think I'd agree with is the WMD part. I don't think a pressure cooker bomb qualifies as a WMD. Besides that, we're talking about a US citizen, committing a horrible crime on US soil. I would not consider him an enemy combatant. He'll get his desevrings going this route.
About the family, it is really annoying seeing how some members appear to be claiming it is a coverup, but I can see why it is happening. I wouldn't want to believe that a close family member is capable of doing something like that either, and I'd be clinging to any hope I had that he/she was not responsible.
When this is all said and done, and the younger brother is tried and sentenced, this could end up being an excellent case study in human psychology. How did this kid get turned into a terrorist? How did he go from being a normal college kid to trying to kill hundreds of people? If we can understand this, which I doubt we'll ever be able to completely, we are one step closer to being able to prevent it, and possibly saving the next kid from turning into an evil terrorist a-hole.