"Siscon" centers on the complex relationship between siblings, primarily focusing on the dynamic between a brother and his younger sister. The narrative explores themes of familial bonds, dependency, and the blurred lines between sibling affection and romantic feelings. The brother, often portrayed as overprotective, exhibits an intense attachment to his sister, which drives much of the story's conflict and emotional tension. The sister, while initially appearing passive, gradually reveals her own complexities, challenging the brother's perceptions and forcing both characters to confront their feelings.

The plot unfolds through a series of everyday interactions and significant events that test the siblings' relationship. External characters, such as friends, classmates, and other family members, occasionally influence the narrative, adding layers of social and emotional complexity. These interactions often highlight societal expectations and norms, contrasting with the siblings' unconventional bond. The story does not shy away from exploring the psychological and emotional ramifications of their relationship, presenting a nuanced portrayal of their struggles.

As the narrative progresses, the brother's overprotectiveness evolves, sometimes manifesting as possessiveness, while the sister begins to assert her independence. This shift creates a dynamic tension, as the brother must reconcile his feelings with the reality of his sister's growing autonomy. The story occasionally delves into flashbacks, providing context for the siblings' deep-seated emotional connection and the origins of their current dynamic.

The manga balances moments of lightheartedness with more serious, introspective scenes, creating a multifaceted portrayal of sibling relationships. The characters' development is central to the narrative, with their evolving emotions and actions driving the plot forward. The story's exploration of familial love, personal boundaries, and emotional dependency offers a thought-provoking examination of the complexities inherent in close relationships.