"My Sister, My Writer" centers around the relationship between two siblings, Itsuki and Chihiro Hashima. Itsuki is a high school student and an aspiring light novel author, while Chihiro, his older sister, is a successful and popular light novel writer under the pen name Yū Nagami. Despite her professional success, Chihiro struggles with writer’s block and relies on Itsuki’s creative input to overcome it. The dynamic between the siblings is complicated by Chihiro’s romantic feelings for Itsuki, which she keeps hidden due to societal taboos and her fear of disrupting their relationship.

The narrative explores the challenges Itsuki faces as he navigates the competitive world of light novel publishing. His talent is recognized by Miyako Shirakawa, a sharp-tongued editor who becomes a key figure in his career. Miyako’s no-nonsense attitude and high expectations push Itsuki to refine his skills, but her interactions with Chihiro often lead to tension, as Miyako is unaware of Chihiro’s reliance on Itsuki’s ideas. Meanwhile, Itsuki’s classmate, Nayuta Kani, a talented illustrator, becomes involved in his projects, adding another layer of complexity to his personal and professional life.

As the story progresses, the characters’ relationships deepen, and the boundaries between professional collaboration and personal emotions blur. Chihiro’s internal conflict over her feelings for Itsuki becomes more pronounced, while Itsuki remains largely oblivious to her struggles. The series balances lighthearted moments with emotional undertones, highlighting the challenges of creative work and the complexities of familial and romantic relationships. The narrative does not shy away from exploring themes of dependency, ambition, and the ethical dilemmas of collaboration, particularly when personal feelings are involved. The interplay between the characters drives the plot forward, creating a mix of drama, humor, and introspection.