My granddaughter fell on the upturned corner of a hard back book when she was around 2. She still has a faint dark line over her cheek bone. She's 10. I don't know why it's lasted so long. It's possible that the cut was deeper than we thought. It was more than a scratch.
I've had several surgeries. One can't get much more of a serious scar than when being cut open. The scars have gradually faded. The scar from the surgery I had in my 30's is barely visible. The last surgery was 7-8 years ago on my chest and it's a faint lighter colored line. It doesn't keep me from wearing lower cut tops. No one seems to notice. Every once in awhile I notice it but it's really no big deal.
I believe that her scar will gradually fade. I hope that you can make peace with it. If you're concerned, you just might be giving her the message that the scar is a negative thing and that she's not pretty because she has it. She is a beautiful little girl! You want her to feel confident that no matter what she is beautiful! I urge you to stop paying attention to it and before you know it, it will be gone. Focusing on it does not make the skin regain pigmentation any faster.
Also, how does it make little brother feel when the scar he caused is the focus of so much attention? At one he doesn't know what's going on and doesn't even remember the incident. However, this has the making of family lore. You don't want everyone talking about what brother did long time ago.