Gimai Seikatsu

Following his father's remarriage, Yuta Asamura meets Saki Ayase: his new stepsister, who also happens to be the prettiest girl in his school.

Very quickly, the two teenagers make a promise to each other: for the sake of their parents and for their own stability, they will make sure not to get too close to each other, not to upset each other too much in their daily lives, while trying to maintain a friendly relationship.

However, none of this is very straightforward. Saki, who has known nothing but loneliness, doesn't know how to rely on others, while Yuta can't figure out how he should behave around her.

Gradually, the duo learns the basics of living together as stepsiblings, until the day they stop being strangers to each other... (source: animotaku.fr)