You are aware that the police didn't find or discover her body, her parents did. right?
Thats why the entire crime scene was compromised, the cops screwed everything up.
Her father found her, but he was not alone at the time. A family friend was with him. Yes the scene was compromised. Yes, the police totally screwed up the handling of this case. But, I think you need a lot more solid proof before you lightly accuse a child(or anyone for that matter) of such a heinous crime.
There is absolutely NO evidence that her brother had anything to do with the murder.