"I Kissed My Girlfriend's Little Sister?!" is a romantic comedy manga that explores themes of love, misunderstandings, and the complexities of relationships. The story centers on a high school student, Yuuto, who is in a committed relationship with his girlfriend, Aoi. Their relationship is stable and affectionate, but a sudden and unexpected incident disrupts their dynamic. One day, Yuuto accidentally kisses Aoi's younger sister, Hinata, during a moment of confusion. This impulsive act creates a ripple effect, leading to a series of awkward and comedic situations as Yuuto tries to navigate the fallout.

Hinata, who harbors her own feelings for Yuuto, becomes more assertive in her interactions with him, further complicating matters. Aoi, unaware of the kiss, remains devoted to Yuuto, but her trust is tested as tensions rise between the three characters. The narrative delves into the emotional struggles of each character, with Yuuto caught between his loyalty to Aoi and his growing awareness of Hinata's feelings. The story balances lighthearted humor with moments of introspection, as the characters grapple with their emotions and the consequences of their actions.

The manga explores the dynamics of sibling relationships, the boundaries of romantic love, and the challenges of communication in relationships. Yuuto's attempts to resolve the situation without hurting either sister drive much of the plot, while Hinata's boldness and Aoi's unwavering kindness add depth to the interactions. The series maintains a focus on the emotional growth of the characters, as they confront their feelings and the unintended consequences of their choices. Through its blend of humor and heartfelt moments, the manga presents a relatable yet exaggerated take on the complexities of young love and familial bonds.