Imouto ga Bun Retsu Shita.
妹が分裂した。 · いもうと が ぶんれつ した
est. Rough estimate based on volume count, reprints and magazine — not an official figure
Overview
After years of barely talking to her big brother, Uiha suddenly wakes up to find that she has a body-double, who is both her copy and her complete opposite. Originally published on Twitter and Pixiv and collated by Kadokawa for print release. Includes an extra chapter.
Source: AniList
External databases: MADB · AniList
How to compare editions, collected volumes and ISBNs / Sources, updates and corrections