A Feather So Black by Lyra Selene

Set in a world of perilous magic and moonlit forests, this seductive romantic fantasy tells the story of a defiant changeling, her cursed sister, and the dangerous fae lord she must defeat to save her family.
In a kingdom where magic has been lost, Fia is a rare changeling, left behind by the wicked Fair Folk when they stole the High Queen's daughter and retreated behind the locked gates of Tír na nÓg.
Most despise Fia's fae blood. But the queen raises her as a daughter and trains her to be a spy. Meanwhile, the real princess Eala is bound to Tír na nÓg, cursed to become a swan by day and only returning to her true form at night.
When a hidden gate to the realm is discovered, Fia is tasked by the queen to retrieve the princess and break her curse. But she doesn't go alone: with her is prince Rogan, Fia's dearest childhood friend—and Eala's betrothed.
As they journey through the forests of the Folk, where magic winds through the roots of the trees and beauty can be a deadly illusion, Fia's mission is complicated by her feelings for the prince.and her unexpected attraction to the dark-hearted fae lord holding Eala captive. Irian might be more monster than man, but he seems to understand Fia in a way no one ever has.
Soon, Fia begins to question the truth of her mission. But time is running out to break her sister's curse. And unraveling the secrets of the past might destroy everything she has come to love.