The main character lives with his two sisters, Isora and Sakuya. He has recently finished high school, and is preparing to go to college. To do this, he’s planning to move out and go into the city. However, his sisters don’t want him to, and they say that there’s a college right nearby that he can go to. However, he insists, saying that he needs to grow as a person, and to do that he needs to get out of his comfort zone and try new things.
However, his sisters really, REALLY don’t want him to leave. So they do something incredibly ridiculous, yet also incredibly effective:
They divide his room in two with iron bars and a jail cell door. They do this while he is asleep.
Now, he’s locked in his bedroom, and he can’t get out. He literally can’t get out, the windows are barred, the room is barred, everything is barred. It’s a miniature jail and he’s on the inside of it. Isora visits him and informs him of something: They’re both in love with him. Not the brother-sister kind of love. REAL love. They were going to confess to him, but he decided to move away, and they just can’t have that, can they? So they built him a cage and locked him in it. They’re going to take care of him for the rest of his life, and they can all have "fun" together. Thus, the translated title of the game: "Yandere Big Sister’s Brother-Raising Diary".
[Edited from Dark Eye Translations]