Journey to the River Sea — Eva Ibbotson

A girl, a jungle, and a missing inheritance...
Sent to live with distant relatives on a rubber plantation in Brazil, Maia must leave her cozy boarding school in London for the unknown wilderness of the Amazon jungle. Accompanied by Miss Minton, a no-nonsense governess, Maia expects to find curtains of orchids and brightly colored macaws. But instead they find the Carters, Maia's evil-tempered aunt and uncle, with whom Maia's life becomes almost unbearable. It is only when she is swept up in a mystery involving a young Indian boy, a homesick child actor, and a missing inheritance that Maia finds herself in the middle of the Amazon adventure she dreamed of.