The Island at the End of Everything by Kiran Millwood Hargrave

Amihan lives on Culion Island, where some of the inhabitants – including her mother – have leprosy. Ami loves her home – with its blue seas and lush forests, Culion is all she has ever known. But the arrival of malicious government official Mr Zamora changes her world forever: islanders untouched by sickness are forced to leave.

Banished across the sea, she's desperate to return, and finds a strange and fragile hope in a colony of butterflies. Can they lead her home before it's too late?