Responses
I'm not sure about the details. But what I know that she have some sort of ability with her eyes that she can see what normal people can't but can't see what normal people can. Can take a look at these chapters :
Chapter 68 : Shows how she sees her brother.
Chapter 82 : Some info and name about her eyes ability.
Chapter 88 : She explains how she sees things.
I don't really know how to explain, so can try check out the chapters. Tehe~ (=・ω・=)

On ch.144, the black mage sister, she asks if shirone's blonde, right? And the brother was surprised and asked if she can see shirone. What does he mean?? I'm confused. Is she blind or something? Did I forget a part of her lore that she's unable to see people or able to see something different in others? Is shirone not actually blonde??