So... is Kiss x Sis finished or not?
NO, it is not finished.
Why?
The anime finished before the manga:
Anime finished: April 06, 2015
Manga finished: September 21, 2021
MangaUpdates contains information about the anime adaptation and indicates that it is incomplete.
Watching Order (by release date)
Here you have all the anime relations we detected ordered by release date, so you can see the reason for this status in context and also use it as watching order.
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Synopsis
Keita finds himself in a complicated situation when his father remarries, giving him two older twin sisters, Ako and Riko.
They've known each other since childhood and share a very close bond, which has evolved into something more than just sibling affection.
As Keita prepares for high school, his sisters become increasingly determined to win his affections and change his mind about which school to attend.
He now faces the challenge of studying for his exams while simultaneously trying to navigate the increasingly romantic and suggestive advances of his twin sisters.
With their parents' tacit approval, Ako and Riko openly compete for Keita's attention, creating a constant struggle for him to maintain a platonic relationship amidst temptation.