Shirokuro Tsukeru?

Two kingdoms exist in the heavens, each with their own royal families. Despite being at war, a prince of the Black Country, Sakuya, is friends with a prince of the White Country, Rui, and his sister Mio. The three spend a lot of time together at a lake on neutral ground. One day, Mio frantically flew to their spot and called out to the boys, but was drawn into a strange portal and disappeared into darkness before saying anything. Left behind, Sakuya and Rui are still drawn to the location. ...