Imouto sae Ireba Ii.

A Sister's All You Need follows Itsuki Hashima, a light novelist whose obsession with little sisters shapes everything he writes and drives him to keep chasing the perfect story built around them. His world is defined by a strange but close-knit circle of friends and fellow creators: Nayuta Kani, a brilliant yet perverted novelist who is in love with him; Haruto Fuwa, a fellow male author whose work has already seen considerable success; Miyako Shirakawa, a good friend he met in college; and Chihiro, his perfect younger step-brother, who handles the housework and cooking at home. Together, they pass their time playing strange games, taking spontaneous journeys, cracking silly jokes, and celebrating each other's successes. Beneath the comedy, however, each of them is carrying private burdens, from the pressure of looming deadlines to traumatic events buried in their past. As Itsuki keeps reaching for the ideal little-sister story, A Sister's All You Need becomes a portrait of writerly ambition, messy relationships, and the odd warmth that comes from finding people who accept your quirks without judgment.