I believe the brother will be charged as an adult per California law.
http://www.losangelesjuveniledefense.com/trying-juveniles-as-adults-in-southern-california/
They will determine this afternoon whether he is fit to be tried as an adult and IMO, they will find he is (he understands the process, is not too young to know what's going on, etc.).
I'm not sure how I feel about juveniles being tried as adults. Under 14 I am definitely against. Over, I guess it depends on the callousness, the history of the perp and if there is cruelty or sadism involved in the crime.
I guess for me, it is complex and not black and white. I agree totally that the brains of teenagers have not developed as adults so that must be considered, but I do not view things like this as "double tragedies", I don't feel sorrow for the perp and I don't think what such perps do is simply a product of their age and immaturity.
I think people of almost every age past infancy/toddlerhood have the capacity for good or evil and have the ability to make decisions and understand right from wrong, as long as they are not intellectually disabled or mentally ill. I do not think the fact of their age propels them to do something bad or prevents them from being able to control themselves, even though they are more impulsive when young.
At the same time, I think minors have a greater capacity for change and rehabilitation than many adults. Their brains are more malleable and even those with evil in their hearts or sociopathic tendencies may be more amenable to change and more treatable, IMO. Thus, I can't agree with throwing away the key in most cases. Kids can do some pretty horrible things they later would never do, after maturity and experience allow for greater understanding. But they also still have choices at a young age.
So, I'm on the fence about the juvenile as adult thing.