Cyber Bullying

க்ர்ஸடேன் 🦁🔥 • pagan. pansexual. pro-choice. almost finished my criminal justice/psych degree. not TTC.
Possible triggers?: self harm, bullying
My siblings, including my 12 year old sister, recently changed school districts. Her name's Kim and we figured her fitting in at a new school would be easy. Which is it. She has no shortage of friends already in 2 weeks. Good kid, good grades. She has an Instagram account which she uses for posting art she does. It is private and you do not need to be friends with someone to message them. This is the only social media my sister has. A girl from her school apparently does not like her and sent her the messages below. My mom posted the Facebook message. 
I am EXTREMELY proud of how my sister handled this. While I wouldn't have kept talking to this kid, I'm glad she did because who knows who else this girl is bullying? On Monday, they're going to the school's office to get this taken care of and the police have been notified. Kimmy has blocked the girl and reported her to Instagram but has saved the messages for the police/school. 
Any other situations with your kids or siblings? 
How serious do you think we should treat kids that do this? 
Is this the parent's fault or the girl's fault?