The Light Between Worlds by Laura Weymouth 

Six years ago, Evelyn and Philippa Hapwell were swept from war-torn England and into a woodland fantasy kingdom. Now back in the real world, eldest sister Philippa is relieved, but Evelyn is grieving for the magical place she called home. When she goes missing, Philippa must confront the truth of what happened to them, and the lengths her little sister was willing to go for a chance to return to the kingdom of her heart ...