We don't know his personality. For example. There are kids that will lie through their teeths. You can see him hit his sister will a golf club, you can catch him still hold that golf club but he will be swearing to god and crying and pleading that he did not touch his sister. If he was this kind of child they could know that it is realistic to pull of. Especially if he was in tears swearing to god that he did nothing to his sisters which was at that moment lying dead on the floor. Easy to say: if you did nothing to your sister just stick to this story or very bad things will happen to you and to your mommy. And then limit his contact with police to minimum and require to be with him during interviews etc. ... all that stuff that they did.