"Kiss×sis" centers around Keita Suminoe, a high school student living with his father, stepmother, and two older stepsisters, Ako and Riko. The series explores the unconventional and often comedic dynamics of their relationships. Ako and Riko, who are fraternal twins, develop romantic feelings for Keita, leading to a series of flirtatious and provocative situations. Despite their persistent advances, Keita struggles to maintain a balance between his familial bonds and the growing romantic tension.

The narrative delves into the complexities of their daily lives, blending slice-of-life elements with romantic comedy. Keita’s attempts to focus on his studies and personal goals are frequently interrupted by Ako and Riko’s antics, which range from playful teasing to more overt attempts at seduction. The series also introduces secondary characters, such as classmates and teachers, who add to the comedic and dramatic tension.

Ako is characterized as the more reserved and academically focused of the twins, often hiding her feelings behind a calm demeanor. Riko, on the other hand, is more outgoing and openly expressive about her affection for Keita. Their contrasting personalities create a dynamic interplay that drives much of the humor and emotional conflict in the story. Keita, caught between his sisters’ affections and societal norms, frequently finds himself in awkward and humorous predicaments.

The plot progresses through a series of episodic scenarios, often highlighting the blurred lines between familial love and romantic attraction. Themes of loyalty, personal boundaries, and the challenges of adolescence are explored, though the series maintains a lighthearted tone overall. The interactions between the characters are central to the narrative, with the twins’ relentless pursuit of Keita serving as the primary source of both comedy and tension.

"Kiss×sis" balances its comedic elements with moments of introspection, particularly as Keita grapples with his feelings and the implications of his sisters’ actions. The series does not shy away from exploring the moral and emotional dilemmas that arise from their unconventional relationship, though it often resolves these conflicts with humor and a touch of absurdity. The result is a story that blends romantic comedy with familial drama, offering a unique take on the complexities of love and relationships.