My little sister has come across a toxic online friend, she's been really close to that friend for a month or two, but Saturday night, her friend got jealous that she was playing with someone else for once, and she said "Hi. This is my last hi to you btw." so my sister asks, "Wdym?" and she says, "I lost friends because of you, I'm going back to Line and Carlie." (Line and Carlie are two toxic friends from the past, who ghosted my sister and her friend for "Space.") so my sister says, "Okay, go ahead, do whatever." then, the friend says, "It's hard to say goodbye to you." so my sister says, "So don't say goodbye." so the friend says, "Well you're kind of replacing me, for playing with someone else.." so my sister says "I'm only playing with my new friend right now, I've played with you for days for the past two months, I want some change just this once." so the friend says, "You've been avoiding me for the past week," which she hadn't. Then, after awhile, she started telling my sister to "End it all" and that she's "Fat" when my sister has been stress eating and such because of her. She threatened to edit chats to look like she's "Threatening her." this isn't the first time this has happened, which is why my sister was so passive aggressive about it. My sister blocked her, and today, she exploited to join my sister, and started saying "Please im sorry," and I've told her to leave my sister alone, as it's not good for her mental health. I recently got a test for the virus (which came back negative) and she said "Hey, I hope you test positive. Now let me talk to ur sister." So they had a little fight, the friend started telling my sister to "Leave life" due to a chat filter, and my sister was laughing at me, because I was trying to keep her from crying, and all of that. She tried scamming my sister in Adopt Me, and said, "I'll leave you alone if you give me all of your pets." and that brings us to now.

Basically, my sister made a friend, who turned out to be using her, and keeps giving my sister threats and things, for no reason. I need help on how to stop this. Thanks. SERIOUS ANSWERS ONLY