Changeling

Teenaged Lucia's family means everything to her—mother, father, grandmother, sister. But when her grandmother, Maggy, and sister, Nara, vanish, Lucia discovers that she and Nara aren't sisters. Undaunted, Lucia chases after Nara and Maggy into their own realm to bring them back home.

In this contemporary fantasy story from Adrienne Wood, who we are isn't always what we've been told, but family is forever.