No, No, No! You can't have it both ways.
Either Burke was an out of control 9 y/o who murdered his sister or he was a typical 9 y/o boy. Which is it? If Burke could be coached in that short period of time to "do the right thing," don't you think John and Patsy would have coached him before and prevented this to begin with? "Hey, don't forget, let's keep this to ourselves, okay?" isn't going to work on a child with serious impulse control issues. I'm thinking they might have mentioned a few times in the past that he shouldn't hit, smother, or otherwise abuse his sister. How'd that work out? Now you think they can just tell him a couple times not to tell anyone what happened, all the while running around like crazy people trying to cover up the murder of their daughter that was committed by their son, and Burke, an out of control murderer would finally get it? You don't get to make Burke out to be a monster who is out of control when it is convenient, but turn him back into a typical 9 y/o when you need him to be "normal" to fit your narrative.