Imouto to Ecchi Suru no wa, Onii-chan no Gimu da yo ne!

A high school boy lives with his younger sister, with whom he shares a close bond. Their relationship takes an unexpected turn when she proposes a secret arrangement that blurs the line between sibling affection and physical intimacy. He finds himself torn between his sense of responsibility as an older brother and the desires that begin to surface.

The situation escalates as they navigate the complexities of their new dynamic, hiding it from those around them. Each chapter explores different scenarios and challenges that test their commitment to the pact, forcing them to confront the true nature of their feelings. The story delves into the consequences of crossing boundaries within a family setting.