reminds me a lot of my early high school years, best advice I can give you is they do it for the satisfaction. They are lacking something in themselves, so they find someone else to make feel bad so they feel better.
How to deal with it is tricky, you sis is at the age where shes trying to find where she fits in. And eventually she will find a group of friends who like her for her. And that helps with the bullying, because then shes not alone. Being alone makes her a easy target for bullies.
The best way to deal with it, is take away the fun for the bullies. Start laughing with them, say something like you know thats kinda funny i never thought about that. Then go back to minding your own buisiness and ignore them, after a few times of it not being fun anymore they will move on.
Now if they are physically bullying your sister, my advice is simply stand up to them. its not fun and its kinda scary, but once you show your gonna stand your ground. That takes away the fun of making someone feel bad so you feel better, also if she cant do that then she needs to talk with a teacher she trusts and see if they maybe can work out a different seating arrangement, or anything that keeps them apart theres always something that can be done. Basically she just needs to show them she isnt scared of them, and that what they say dont bother her. And they will move on to someone else to pick on. Thats just how it works, and when she sees them picking on someone else, befriend that person. Before she knows it she will have friends she never thought she would have, and there is always safety in numbers
Keep your chin up kiddo, good for you looking out for your sister.
TCG