That is an interesting question, topcop.
Here is my theory of the crime, which proposes that BR staged the wrist and neck ligatures by himself:
http://www.websleuths.com/forums/showthread.php?20426-Members-Theories&p=11899603#post11899603
My theory of events supposes that a coverup by BR to avoid punishment was not only possible, it was within the expected framework of moral development for a child his age - see work by Lawrence Kohlberg, good short explanation here:
http://pegasus.cc.ucf.edu/~ncoverst/Kohlberg's Stages of Moral Development.htm
I am not sure most adults understand or recall the very different moral code of children. We tend to underestimate children's desire to avoid punishment and their egotistical reasoning. We romanticize them, forgetting their capacity for cruelty via self preservation. Sure, some children develop higher level moral reasoning early, but most move through the stages at the expected rate. The important point is this: Immature moral reasoning is NORMAL in children. So though this may have been an extreme case of a child avoiding punishment, morally it must be forgiven. And legally, it must be forgotten.
In my theory there is no legal case to be found, and there hasn't been for many years. If the DA had any inkling of the actual innocence of this boy's actions (i.e. he was covering up the head bash by himself), he might have been more inclined to shield him. A case against the parents was destroyed in the first hours at the scene and thus their lawyers could argue IDI in court forever. If the DA knew that the preponderance of evidence was BDI and it was an accident, then why bring the circus to town? Why try the case? To accuse the mom with ovarian cancer of writing a fake note? The Grand Jury returned the exact indictments needed for a case against the parents as accessories after the fact, but the only way to prosecute was to name the underage son as the "killer" by accusing his sick mom of writing the note, and still probably lose. There was nothing to gain. It was all the more tragic that a child apparently killed his sister trying to save himself from punishment, thinking she was already dead. If true, it is heart breaking.
This DA theory, by the way, applies to Alex Hunter. I think Mary Lacy drank the IDI Kool-aid.