Hatsumono no Imouto wo Jikkei ga Okasu!

A young girl is about to celebrate her entrance into a new school, and her four older brothers have prepared a special celebration for her. What should be a warm family gathering quickly takes a dark turn as the brothers' intentions become clear.

The celebration is merely a pretext for the brothers to impose their twisted desires on their sister. Under the guise of familial affection, they begin to systematically take advantage of her, claiming her first experiences as their own. The girl finds herself trapped in a situation where the line between love and abuse is blurred, and she must navigate the disturbing dynamics of her own household.