[Always *]

I can understand how your sister feels. I have a big scar on my wrist from an injury and people used to say it looked like a suicide scar which was kind of awful but it was clearly a surgical scar and was just a terrible thing to say any how. IT make me really insecure about my wrist for a long time. She's probably dealing with a lot between getting over the injuries especially if they still and repercussions of what happened to get the injuries. It's probably making it harder to be getting bulied too.
You might want to see if she can go talk to someone as well.
Unfortunately, as someone who's also been bullied for various things I can also completely understand why you'd be upset by this knowing your sister is going to feel so awful and that you can't even stop it. You could always offer to help her choose out some light (and heavy) shirts if she is feeling insecure about the scars so that she can cover them up easier.
If it's any reassurance people usually shut up about these sorts of things in the long run. Unless her face is seriously disfigured (and you said it's just her arm and leg) then she'll be fine. If she had scars all over her face she is still likely to have some people notice, but yeah. Eventually her peers will have the good sense to not say anything that would be rude or hurtful.
The best thing to do in this situation would be to think about her, think about helping her and trying to find ways to alleviate her depression over her injuries. I know it's a trigger for you, but she's the one who has to live it and helping her will help you inevitably