"Imōto wa Dōjin Shōji Cosplay Kei" centers on the relationship between siblings and the world of cosplay and doujinshi creation. The protagonist, Yuuya, is a high school student who discovers that his younger sister, Rinka, is a popular cosplayer and doujinshi artist under the alias "Rin." Rinka’s work has gained significant attention online, and she is known for her elaborate costumes and creative content. Yuuya, initially unaware of her activities, stumbles upon her secret and becomes involved in her world.

The narrative explores the dynamics between Yuuya and Rinka as they navigate the challenges of balancing their personal lives with Rinka’s growing popularity in the cosplay community. Yuuya, despite his initial reluctance, begins to support Rinka by assisting her with her projects, including costume creation and event planning. Their collaboration deepens their sibling bond while also exposing them to the complexities of the doujinshi and cosplay industry, including fan expectations, competition, and the pressures of maintaining an online persona.

The story also introduces other characters who play significant roles in Rinka’s career, such as fellow cosplayers, doujinshi creators, and fans. These interactions highlight the collaborative and competitive nature of the community, as well as the creative and personal challenges faced by those involved. The series delves into themes of identity, self-expression, and the balance between public and private life, particularly in the context of internet fame.

Throughout the manga, the focus remains on the evolving relationship between Yuuya and Rinka, their shared experiences, and their growth as individuals. The narrative combines elements of slice-of-life, comedy, and drama, offering a nuanced portrayal of the cosplay and doujinshi culture while maintaining a grounded exploration of sibling dynamics.