Domestic rape gang rape raped by stepbrother

Natsuki's mother remarried and three stepbrothers were born. Everyone was kind and I felt like they had lived for a long time... Except for one person. His third son, Ken, had so much thought of Natsuki as the opposite sex, rather than as a family, that he had lingered in his room and avoided Natsuki. Then, one afternoon, Ken finally pushes Natsuki down. However, at this time, Natsuki had no idea. This was just the beginning of the collapse...