The Island at the End of Everything by Kiran Millwood Hargrave

There are some places you would not want to go. Even if I told you that we have oceans filled with sea turtles and dolphins, or forests lush with parrots that call through air thick with warmth… Nobody comes here because they want to.
The island of no return.

Ami lives with her mother on an island where the sea is as blue as the sky. It’s all she knows and loves, but the arrival of malicious government official Mr Zamora changes her world forever: her island is to be made into a colony for lepers. Taken from her mother and banished across the sea, Ami faces an uncertain future in an orphanage. There she meets a honey-eyed girl named for butterflies, and together they discover a secret that will lead her on an adventure home. Ami must go back to the island of no return, but will she make it in time?