The Forgotten Sisters is the third and final book of the Princess Academy trilogy, published in 2015. It focuses on Miri's struggles as a tutor to the king's cousins, who live in the swamp lands.
After a year at the palace, Miri and her friends are ready to return home. But when the king orders Miri to start her own princess academy in a faraway swamp for three royal cousins, she is utterly dismayed. And Miri's new students are not what she expected. Astrid, Felissa, and Sus are more interested in hunting, fishing, and wrestling than learning about etiquette and history, and they know nothing about their royal ancestry.
As Miri spends more time with the girls, she starts to suspect that they are part of a long-buried secret, and that the key to solving it rests in her and the sisters' hands. With her new friends at her side, Miri must gather all her strength to solve the mystery- and finally make her way back home.