Our girl better not join the band now. In fact, this girl should regret her actions and then be forgotten. Such actions shouldn't be treated lightly. Because she wants the FMC to join her band, the FMC will join? Stupid trope. If she doesn't want to do it, just respect that.
And her unreasonableness should not be rewarded. Maybe if this was done in a more nonsensical story, I'd give it some room... but this story takes itself too seriously at times, so this kinda twist won't work. It's just annoying.
@Nihilist1 Wrong, in Nisekoi, at least the series itself was more episodic. It wasn't about love triangles or finding the secret person. Those were just MacGuffins for the series. It was a collection of stories about the characters meant to be enjoyed on their own with story elements for the series sprinkled just to give a clear ending.
This on the other hand is actually about the love triangle between 2 sisters and their childhood friend. It's completely different, even if they make this a harem.