I don't think minors should ever be charged with crimes in adult courts. Radical, I know. But you know what, we don't go around making random exceptions for children and adolescents to drive, drink, smoke, vote, gamble, join the military or anything else. Why should crime be the only thing that doesn't work this way?
This isn't to say a 17 year old serial killer should be allowed to be walking the streets. It just means to let a family court deal with them and when they're older if they're beyond help they can be institutionalized in a mental health facility or a prison indefinitely.
These are usually ones at hospitals, because they are understaffed and have a whole host of people strung out on drugs and violent, unstable criminals too close to suicide attempt victims and the staff just doesn't have the resources to help.
Most mental health facilities, long term ones though, have voluntary patients as a majority and it's not like stereotypes show. There are exceptions, sure. But in any case, two disturbed twelve year olds should not be denied a chance at living freely again someday if they can be rehabilitated, which we don't know.
Frankly I wouldn't want the girls back on the streets...