11 year olds 'sexting'

I went on the computer at my mom's house and when I signed in, a program called Discord popped up and it had a bunch of chats that was signed in by my 11 year old sister's friend. I could not believe my eyes! It was a bunch of role play explicit sex chat with a boy. Some were even group chats with her and 2 boys who were 'gang banging' her. The amount of detail this girl was describing, theres no doubt shes still a virgin. I felt dirty like seeing CP on my computer the way she was talking about letting these boys cum on her face and so on. My mom doesnt want to tell her mom because last time she told her mom about facebook posts saying she wanted to kill herself, the mom brushed it off. I dont think my sister should be allowed to talk to this girl or the boy anymore and I want to tell her mom but my mom wants me to let it go! Its eating me up inside. If this girl is still a virgin, she wont be for long and I think shes been sexually harrased because chats with her other girlfriend's she descirbes these role play sex chats as ' going to get raped' What world am I bringing my daughter into??? Im at a loss what to do... EDIT: So I know its sneaky but we left he discord signed in. They are still talking but not allowed to hang out in person. its my moms choice, so be it... but seems like stuff is or has happened to her n her mom doesnt care. There was more about a 20 year old stalking her but I dont know how much of what she said is real and whats her trying to get attention from the boy that she was sexting with because he stopped talking to her, which she does alot...