I have no doubt that the little boy had no idea of what would happen. His parents are completely responsible for the homicide and should be sufficiently punished... I just hope that this issue is properly handled with the boy. Can you imagine growing up, knowing you killed your sister? The guilt would eat me alive.
Not only that, but also friends, neighbors, other members of the family, all of them knowing that you killed your little sister. The stress and pressure will be immense for this little guy, and it breaks my heart that he will live the rest of his life this way.