The Middler by Applebaum

Eleven-year-old Maggie lives in Fennis Wick, cut off from the outside world by a boundary beyond which the seemingly endless Quiet War rages and the dirty, dangerous, and deceitful wanderers roam. Her brother Jed is an eldest, revered and special, a hero who will soon go off to fight in the war. But Maggie’s just a middle child—a middler who's invisible and forgotten, even by her own family.

One hot September day, when she chances upon a wanderer girl in hiding, Maggie decides she wants to be a hero like her brother and sets out to capture the intruder. But once Maggie peeks past the hedges of the boundary for the first time, suddenly everything she’s ever known about her isolated town gets turned on its head. Perfect for fans of Pax, Orphan Island, and Beyond the Bright Sea.