Until We Become a Family

"I'm a 30 year old, unmarried man who lives with his lover. ...Well, that was the case until yesterday." Oujiro is a worker who lives with his lover, but for some reason, keeps the fact a secret from his coworkers. Out of the blue, his sister and her children come and visit him after 10 years. He reluctantly lets them sleepover after his sister up and declares,"We're staying the night." But in the following morning, he wakes up to find her gone. The only thing she left behind were her two children and some change—