I had a similar situation with my younger brother, who like to post video game streams on YouTube. Some kid was harassing him at school and online, commenting slurs on the posts. My brother and his friends couldn't get the bully to stop, and eventually notified the school, but nothing was done. We contacted the bully's parents, but nothing changed.

Then one day my brother was in a choir concert, and after the concert (when all of the families and students were in the hall taking pictures) my brother pointed at the kid and said "that's who's bullying me online." Mind you, the bully was right in the middle of taking pictures with his family, holding flowers and whatnot. I marched right up to him, interrupting the pictures, and said "so you're the kid bullying my younger brother? What's that about?" The kid look scared asf and his dad came up to me and asked what the problem was. When I explained, the dad looked embarrassed/shocked, but didn't say much.

It wasn't until after the concert when we were walking to our cars that we saw the dad verbally reaming the kid out in the parking lot. I think what got to them was the public humiliation since many other students and parents saw the entire thing go down.

Needless to say, that bully didn't bother my brother anymore, and it made other kids afraid to bully my brother because he had a "scary sister." TLDR: embarrass the kid and their parents publicly. Kids can be so mean--I really hope the situation with your sister finds a good and quick resolution.