So... is Ore ga Suki nano wa Imouto dakedo Imouto ja Nai finished or not?
NO, it is not finished.
Why?
The anime finished before the manga:
Anime finished: December 19, 2018
Manga finished: March 19, 2020
MangaUpdates contains information about the anime adaptation and indicates that it is incomplete.
Watching Order (by release date)
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Synopsis
Yuu, an aspiring author constantly facing rejection, discovers that his talented younger sister, Suzuka, is a successful novelist using a pen name.
To protect her secret, Suzuka asks Yuu to pretend to be her.
Yuu agrees, hoping to use this opportunity to improve his own writing and eventually publish his own novel, all while navigating the challenges and relationships that come with his assumed identity.