An Invisible Bully?

Okay so my sister is having a teensy bit of a problem with her friend Alaine, but here's the catch everytime they speak with each other it's really her cousin, _________, on her sn instead of Alaine herself. Her cousin likes swearing a lot and threatens my sister that if she doesn't stop being "cheesy" she'll come over to her school and beat the s*** out of her. On these sorts of topics i'd either tell my sister that her cousin had a bad childhood or thinks that's she's really "cool" or powerful just because she's a senior in high school. But she can't just tell my sister NOT to be who she really is---i meann it's like asking your grandparents not to wear their fake teeth when they eat!! I just think it's really wrong--disturbingly wrong. What should i doo? i've done almost just about everything i could in my power. HELP!!