Shirokuro Tsukeru?

Two opposing kingdoms rule the heavens, each led by their own royal family. Despite the ongoing war, Prince Sakuya from the Black Country maintains a friendship with Prince Rui and his sister Mio from the White Country. They often gather at a lake in neutral territory. One day, Mio arrives in a panic, calling for the boys, but is suddenly pulled into a mysterious portal and vanishes without explanation. Sakuya and Rui remain at the site. Three years later, a girl who looks exactly like Mio falls from the sky, wearing unfamiliar clothing. She insists she is not the same person and claims to be from a lower realm.