I wanted to make a comment on this since I'm noticing some comments about an "ultimatum" being made.
I always find these kind of ultimatum really dumb?
Disband if you don't win ? How about no ? You don't deserve to be with her? It's not your decision to make, just go away?
Those kinds of antagonist really only work if they have actual power to force bad consequence on the protagonist, but she's just full of hot air.
Hope next time Haru and Miyuki meet that Haru is just like "I didn't agree to any of this shit" in regards to them disbanding if they lose. Like, what the hell is in it for her to make that deal? Hell, she has no incentive to honor Miyuki telling her not to tell Yuu either. She should just tell her that her sister is antagonizing her for no reason.
First I want to apologize if my wording has caused confusion. I've went ahead and re-worded it to attempt to make the line more clear and re-uploaded the page.
In this line, Miyuki was more saying that if they're not good enough to pass the audition, then they shouldn't even bother to continue as a group. Miyuki sees the audition as a very low level. She's not directly telling them to disband if they lose, since at this time there's no competition. She simply sees it as "why would I need to prove I'm better if you do it for me and fail at the first hurdle." She tells them to "have some dignity" because she'd likely rather not see her sister fail to do something so basic. She's kinda just being rude, but not trying to control them or demand things of them.
Miyuki is almost certainly not going to be participating in this audition. It's her mother's company (yay nepotism) and she's been training as a singer under RURIKO all this time already and has been undergoing professional training since she was a child. More likely, she's like the "Pokémon Champion" and waiting at the peak of the summit for them as a final test once they've made it through all the trials.
All we know for sure is that Miyuki seems to idolize Yuu and her singing, and the reason she sees herself as more deserving is because she's had more training, while she considers Haru's singing flawed. Essentially, she thinks her sister deserves only the peak of perfection as a partner and is not seeing what our husky brings to the table.
Anyways, please invest my delusion of Miyuki x Hikari, because if anyone can melt this stuck-up ice queen it's that ball of sunshine.