My sister is "anti" and I really need help

First, I'll start by apologizing because I feel like I might be making a mistake somewhere. I don't know much about it all, my activity in fandoms is limited to looking at fanarts and reading HP and Naruto fanfics on ao3. All I knew about the whole discussion so far was that it exists. But my parents know and understand even less, and we are all stressed right now.

Basic information: my parents are over 40. . Basically, my mom only understands Facebook and sometimes watches TikToks made by other women over 40. Dad mainly watches sports on YouTube. I'm 21. My younger sister "Olivia" is 15. We live in some country in europe for three year now.

My sister is very interested in fandoms and fanfictions, she's the one who tells me all the news about new series, books, anime, etc. Sometimes she got a little emotional about certain topics, but until now nothing worried me.

At the end of November, my parents had to go to school because my sister was in a Christmas-themed play and there was a meeting to discuss the details. That was the first time her teacher talked to them about her behavior. He said she often makes strange comments to her friends - one boy and two girls. He said that at first it seemed like regular teenage teasing, but then one of the girls came to him and said that my sister was calling her in an offensive way. She didn't feel comfortable with it, but when she asked my sister to stop, Olivia said no and continued this behavior.

At that time my parents didn’t involve me and tried to talk to my sister themselves, but things only got worse from there. Before, Olivia's friends would come over regularly, but around mid-December, only the other girl ("Emily") started showing up. And every time it ended in some kind of argument.

One day it was just me, Olivia and Emily. My parents were at work and I was in the kitchen when Emily ran down the stairs and go out of the house cursing at my sister. When I asked what happened, Olivia said they "just had a fight", but only a few hours later my parents got a call from Emily parents. Olivia destroyed her sketchbook because "it had inappropriate stuff". My parents tried to talk about it but they didn't understand much, so they ended up sending me. Olivia says that she and her friends have become very interested in the anime "jujutsu kaisen", but "Emily and Agatha (the other girl) are too interested in the main characters." and this makes Olivia feel uncomfortable. Then she started explaining the whole "anti/pro" thing to me in more detail. She told me that in her opinion "Emily and Agatha pretend to be anti, but they are pro".

She told me that Agatha was a "lost case" for her and started listing all the characters Agatha liked from different anime, but "Emily is her friend and she would like to fix the situation.". Except that "Emily never listens.".

After that conversation, I did some more research from other sources about pro/anti. I'll admit I visited the subreddit for both, and while I still feel like I'm missing something, I know I don't want my sister to be involved in this. I also tried to explain to my parents what I found out, but their reaction was even worse than mine. They are basically lost.

I'll admit that we didn't do much because of this. My mother only had a conversation with Olivia about how she didn't have to be friends with these girls and that she should back off a bit. She also tried asking for help at school, but that also ended up being just one conversation between Olivia and the teacher.

Which brings us to Friday, January 31st. When I got home, my sister was already there. My parents were already there too and my mom was crying. Olivia attacked Agatha. There were no serious injuries, but Agatha's parents are furious. They're basically threatening us with the police, and Emily's parents joined in on Saturday, adding more things we didn't know about. They said that Olivia was stalking Emily online, on various sites and actively encourages others to attack her. They sent us some screenshots - from reddit, twitter, and even ao3. It looked really bad, honestly.

Olivia doesn't want to talk to us. In her opinion, "she's already said everything" and "we should understand by now that she was just defending herself." Today Olivia refused to go to school and practically fought with our mom and called her "disgusting.". Now she's locked in her room and still refuses to talk.

Mom has a talk at school tomorrow, she also wants to find a therapist but in our country it will be a long wait. She also took her laptop, but we have no idea where Olivia hides her phone. Dad basically tries to cut himself off from everything, and I really want to help in some way. But I don't know how.

I will be grateful for any advice or suggestion.