@Mizukichama
Putting the matter of the tags aside, Hiyama is not a rival, he's the main love interest. The manga is even described as a "boy x sister love comedy" by the publisher (on twitter), and it's obvious since most of the interactions are between the boy and the sisters rather than between the sisters themselves. Comparing this to Ouji-sama Nante Iranai makes no sense since Ouji-sama had the focus on the girls' relationship while this one focuses on the relationship with the guy. There's a reason Ouji-sama Nante Iranai is tagged as 百合 on Comico while this one is not.
Putting the matter of the tags aside, Hiyama is not a rival, he's the main love interest. The manga is even described as a "boy x sister love comedy" by the publisher (on twitter), and it's obvious since most of the interactions are between the boy and the sisters rather than between the sisters themselves. Comparing this to Ouji-sama Nante Iranai makes no sense since Ouji-sama had the focus on the girls' relationship while this one focuses on the relationship with the guy. There's a reason Ouji-sama Nante Iranai is tagged as 百合 on Comico while this one is not.