How can i report internet verbal abuse?

my little sister is getting called really nasty things by this one girl who she doesn't know is, this one girl starts the entire thing by messaging her and saying 'B****, w**** and most of all the biggest SL*T' she retaliated, by calling the girl rude things back, i'm wondering if i can get this girl in trouble without my little sister getting into anything either by retaliating.