[Heretic]

I have numerous significant, highly visible scars from surgery, so I understand to an extent what your sister is experiencing. For a long time I was embarrassed and even horrified by how it looked.

If it's possible (and, weather permitting, practical) for your sister to cover up some or all of the scars with clothing (long sleeves and long pants for example), that might help her avoid rude stares and ignorant, boorish comments. Beyond that, you could talk to her and tell her that you'll support her through this. She has no need to respond to other people ostracizing her about this. It's none of their business, and she has every right to make people aware of that, either by ignoring them or by telling them flat-out that they're being incredibly rude.

Last but not least, if this is mostly focused on negative attention at school, she can talk to her teachers about it. Better yet, if you're able to, go with her to the school and talk to the teachers. Express your concerns. Make it known that this has become a major issue and needs to stop immediately.

One final note. My scars are between ten and twenty years old and still highly visible. As I've gotten older, they've gone from unwanted attention items to being mostly ignored by other people. If someone does ask about them, I give them a short, vague answer as to their origins.