"Saikin, Imōto no Yōsu ga Chotto Okashiin Da Ga" centers on the relationship between two siblings, Yuichi and Mitsuki. Yuichi, a high school student, begins to notice peculiar changes in his younger sister Mitsuki’s behavior. Once a reserved and distant girl, Mitsuki suddenly becomes overly affectionate and clingy toward her brother, often behaving in ways that blur the boundaries of sibling propriety. This shift in her demeanor creates a series of comedic and awkward situations as Yuichi struggles to understand and navigate her newfound behavior.

The narrative explores the dynamics of their relationship, with Mitsuki’s actions often leading to misunderstandings and humorous scenarios. Her exaggerated displays of affection, such as insisting on sharing a bed or making bold declarations of love, are juxtaposed with Yuichi’s attempts to maintain a sense of normalcy. Despite her antics, Mitsuki’s intentions remain ambiguous, leaving Yuichi and the audience to question whether her behavior stems from genuine feelings or is merely a phase of sibling rivalry or attention-seeking.

Supporting characters, including classmates and friends, occasionally add to the comedic tension, reacting to the siblings’ unusual interactions. The story balances lighthearted humor with moments of introspection, as Yuichi reflects on his role as an older brother and the complexities of familial bonds. The series maintains a focus on the evolving relationship between the two siblings, blending comedy with subtle emotional undertones.