What Should I Do If I Suddenly Have an Extra Sister?

After time travel, the male protagonist originally planned to be a copycat. Because his parents were divorced, the male protagonist lived with his father who was often away from home. Suddenly one day he received a message from his father that he was getting remarried. The male protagonist rushed to the wedding venue after school, but met the female protagonist while taking a taxi. The only people at the wedding were the father, the stepmother and her daughter. In a private room called the Regent Hotel, there was no grand scene of a wedding. The two families had a simple dinner, but there was no peace at all at the dinner table. Not only did the male protagonist find that he suddenly had a stepmother, but what made the male protagonist even more unexpected was that the stepmother's daughter turned out to be a girl from junior high school, and she was the girl who shared a taxi with him. This novel and its characters are purely fictitious. Any similarity is purely coincidental and should not be imitated.