My Sister, My Enemy (EN) (LN)

Kei Kazuki is a Game High School maniac. He always wins international tournaments, even against cheaters he can.

But there is a top class player who has not been known to exist. With his nickname "Lord Mercy". We swear to uncover who he really is.

Kei's arrival of his younger sisters hasn't seen each other since our parents' divorce. Rena's appearance was very different since he was a child. One day Kei found out who exactly the person he was looking for so far was his own younger brother.

The story gets more complicated when Kei learns that a group of people are making use of his sister.