So... is Oniichan no Koto nanka Zenzen Suki ja Nai n da kara ne!! finished or not?
NO, it is not finished.
Why?
The anime finished before the manga:
Anime finished: March 27, 2011
Manga finished: September 16, 2016
MangaUpdates contains information about the anime adaptation and indicates that it is incomplete.
Watching Order (by release date)
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Synopsis
The anime follows Nao, a middle school girl who is very attached to her older brother, Shuusuke.
She desperately wants him to see her as more than just a sibling, and often teases him in the hopes of sparking a romantic connection.
Her life takes an unexpected turn when she discovers a secret about her family history, which only intensifies her feelings for her brother.
However, Nao's dreams of having Shuusuke all to herself might be challenged by outside forces, leading to potential conflicts and comedic situations.