"Idol Sister" centers on the life of a young man named Haruto, whose ordinary routine is disrupted when his long-lost sister, Aoi, suddenly reappears after years of separation. Aoi, now a rising idol in the entertainment industry, moves back into Haruto's home, bringing with her the chaos and glamour of her idol lifestyle. The narrative explores the challenges of their rekindled sibling relationship as Haruto navigates the complexities of living with a celebrity while balancing his own personal struggles.

The story delves into the dynamics of family, identity, and the pressures of fame. Aoi's return forces Haruto to confront unresolved feelings about their past, including the reasons for their separation and the emotional distance that has grown between them. As Aoi juggles her demanding career with her desire to reconnect with her brother, the series highlights the sacrifices and compromises required to maintain relationships in the face of external pressures.

Supporting characters include Aoi's manager, who often mediates between her professional obligations and personal life, and Haruto's close friends, who provide comic relief and emotional support. The series also introduces rival idols and industry figures who add layers of conflict and competition, further complicating Aoi's journey.

Themes of perseverance, self-discovery, and the importance of family are woven throughout the narrative. The story balances lighthearted moments with deeper emotional arcs, creating a multifaceted portrayal of the idol industry and its impact on personal relationships. Through its exploration of sibling bonds and the pursuit of dreams, "Idol Sister" offers a nuanced look at the intersection of fame and family.