I haven't read through the this whole thread. But, did the story not say that it is the girl's 'Foster' parent?
Now, I hope nobody takes this the wrong way, because I know this does not apply to every case... however,
Unfortunately, many foster kids come from troubled and abusive homes, and through their past experiences have learned to conjure up allegations like this. They know how to use this as a way to get action, attention, or whatever it is that they want... ie. 'payback for a teacher they may not like', etc.
Most Foster parenting training programs warn the prospective Foster parents themselves to be very aware of this, to be VERY vigilent and to avoid even the slightest hint of impropriety, because they are at risk of being falsely accused of some kind of abuse or molestation by their foster kids.
Some foster kids have become foster kids because they were indeed abused. They grow up in that respect WAY to fast. And, they know the drill all to well. They often are known to actually instigate inappropriate sexual activity.
It sounds to me like he girl may have actually been motivated by the idea of PROVOKING her foster Dad with this allegation, to see how he reacted, and if he would actually believe her and defend her. Warped, I know, but very very possible in this kind of situation. And, very possible in the reasoning of a troubled teen mind.