I think there might be a civil suit under intentional infliction of emotional distress. I don't know if they would be able to establish the causal link of her death to the harassment, but it is at least a possibility.
Intentional Infliction of Emotional Distress
Also, if the cops wanted to pursue some type of criminal action for harassment, I would be surprised if there was not some statute allowing for that.
The focus seems to be on whether the adults actions caused her death. I think that is the most tenuous part of any case, but I think everyone can acknowledge that their actions caused a certain level of emotional distress and pain for this girl. The family could still recover for the pain caused to their daughter even if a jury does not find that the actions of the adults were the cause in fact or legal cause of her death.