To Me, The One Who Loved You

Hidaka Koyomi is a boy who lives with his father after his parents divorce. He meets a girl named Satou Shiori at his father's research lab, and they begin a relationship that is slowly blossoming into love. But everything changes when their parents decide to marry each other, and they decide to move to a parallel world where they are not stepsiblings.

(Source: Anime News Network)