Any advice about me and my sister? We're being cyberbullied?

Well. Theres this girl who is so hateful and rude, she lies and goes after younger kids to pick on. She says shes 13, but shes seventeen turning eighteen. And theres this boy, hes 10 or 11, he haz issues, he has way to much time on his hands. So i think hes depressed or something but he always cusses, sometimes words i dont even know... He has taught my sister many words that an 11 year old should not know. So anyways. This is mostly my sister under attack, earlier this evening, that eleven year old was talking about s*x, and my sister asked why are you guys talking about that? He said why do you wanna know and other stuff... Then my sister was called a retard, a *****, a c*nt which is unexeptable... She was called lots more i just dont remember. Oh, the only reason they still have contact with each other was through facebook on those stupid group where you can. Chat with everyone... So. Then that teenager budds in and goes like kacie(my sis) u are a ***** u dont deserve to live so stay out of this group you can go to hell. You and your sister were just assholes so go f off. That made me upset and realize we were victims of cyberbullying. Yeah that teenage grl is harsh... She posted stuff about me on youtube saying how much of a ***** i am and an asshole and my sister too and sent me hate mail until i blocked her... Unfortunately i can still talk to her even if she is blocked through those groups. Shes rude hateful a liar and just straight up mean... I dont interact with her anymore but unfortunantely since my sis talks to those groups, the.bullys see her and imidiently start being hateful and rude. Russel, the 11 year old boy called my sis ugly fat a b*tch an *sshole a c*nt a v** fingerer told her to go f herself and shayna the teen did the same exact thing... The hatred is mostly going torwards my sister so does ajyone know how to get this to stop? Shes not taking this well. She crys alot because of these people. Help please? Were desperate. Thanks so much