Maybe without easy access to a weapon, it wouldnt have happened at all. I dont think anyone should discount the immaturity of a child's brain. Absolutely not in a crime like this one. Take for example the 12 year old who shot his pregnant step mother. Or the 12 year old who shot his Neo Nazi father. Kids get raging and angry, most especially imvho, a tween boy. They cant handle it and depending on the kind of child they are, they will off load it in a specific way. Maybe she was teasing him, maybe there was a slap fight that got out of hand. Maybe he was supposed to go to the game but she was sick and he had to stay and watch her or he was being punished..I dont know what triggered him but at some point he decided to get a knife and end it. And it wasnt enough to stab her once-he was still driven by whatever caused him to do it (I think clearly it was anger) until something stopped him. Maybe the knife broke or he finally came to his senses. And then he lied about it...kids lie when they know they are going to get into trouble.
To me, from my 30K foot view, this is a sibling squabble magnified to the millionth degree and gone terribly terribly wrong. And I think there is a possibility that he was on a stimulant of some kind because the dx du jour for kids these days is ADD or ADHD. (Which doesnt mean, of course, that kids arent appropriately diagnosed and treated for those challenges.)
From an outsiders point of view, there was a lot going on in this large blended family.