My younger sister is addicted to this social media website where shes being bullied ?

[Thomas]

If she is being treated like that on that game and she stills enjoy it, then she must *really* enjoy it and you might have a lot of trouble taking her off it. The solution might be to understand what's causing the bullying and help her put a stop to it.

People rarely ever bully people without any reason. I'm not saying it's justified but no one takes someone at random and thinks "Yeah sure let's all bully her". Though it might be a touchy subject you might want to talk to her and try to see why they're bullying her exactly.
Maybe she should also consider changing server or whatever kind of subcommunity this game offers, at least to make her life easier.

If the bullying can't be stopped, then all you can do is support her and try to interest her in other things, show her she doesn't have to go through ullying and insults to have fun, and that there's other things just as fun as online communities without all the insults.