Takeshi's mother, Shino, and sister, Shinobu, are not blood-related to Takeshi. Although Shinobu attaches herself to Takeshi, she seems to refuse her mother. Except for that, they are peaceful family at first glance. However, there's a secret in this family. Takeshi is taken into Shino's room at night, and he is forced to do sex with her ignoring his will. Because he doesn't want Shinobu to look down on him knowing their relations, he has to obey.

Source: AnimeNfo