First, I thought this manga was treating the sister like "Oshi no Ko" treats Ai
(Ai is like the poster girl, even though she died in the very beginning of the story)
Then I realized my stupidity, 'cause Ai is still super relevant to the story (everything is happening because of her, or for her), while in this manga, the sister is literarly part of 1 or 2 arcs, then ignored. Her actions didn't cause any consequences, so the manga's name could even be changed, and just say the "sister arc" is just a prologue or smth
P.S.: you could say, I'm wrong cause the sister will appear again, but "who cares?", it's been long since we saw her, and future chaps can be ignored, cause they don't exist yet
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tl;dr: tried comparing the sister's and Ai's treatment in their respective stories, then realized my stupidity