One thing stories like this are light on, is details.
I mean how does this happen. People don't just turn aroind and decide to do something like this, there is always indicators, and parents just ignore them. Did the parents have some sort of duty of care for this young girl that included monitoring the 18 yo's behaviour and history in context?
This is what happens I guess when there is little parental involvement and supervision and a lack of anything resembling a proper sexual education.
But dont worry, its all too hard.