"Onee-san no Jijou" centers on the daily life and relationships of its protagonist, a young woman named Aki, who takes on the responsibility of caring for her younger brother, Hiroshi, after their parents pass away. Aki, a college student, balances her academic pursuits with the challenges of raising Hiroshi, who is in middle school. The narrative explores the dynamics of their sibling relationship, emphasizing Aki's dedication and Hiroshi's growing awareness of her sacrifices.

The story delves into themes of family, responsibility, and personal growth. Aki's struggles to provide for Hiroshi while managing her own life highlight the pressures faced by young caregivers. Hiroshi, on the other hand, grapples with feelings of guilt and gratitude as he becomes more aware of Aki's efforts. Their interactions are marked by moments of warmth, tension, and mutual support, reflecting the complexities of their situation.

Secondary characters, such as Aki's friends and Hiroshi's classmates, add depth to the narrative by providing different perspectives on family and responsibility. These characters often serve as foils or sources of support, further enriching the story's exploration of its central themes. The manga maintains a grounded tone, focusing on realistic portrayals of everyday challenges and the emotional bonds that sustain its characters. Through its portrayal of Aki and Hiroshi's journey, "Onee-san no Jijou" offers a thoughtful examination of the sacrifices and rewards inherent in familial caregiving.