Considering the story's content, what fierceteeth thinks was unfair regarding her little brother is that "her little brother grew up off the NightWing Island."
In the Story titled Wings of Fire Wiki, Fierceteeth has a little half-brother known as Starflight.
Throughout the story, it was revealed that Fierceteeth considered Starflight growing up off Nightwing Island as unfair. This makes her jealous of her little brother.
However, Fierceteeth wished that someday Starflight could be reunited with his tribe.
Hence, in this case, it is concluded that the correct answer is "her little brother grew up off the NightWing Island."
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