@greatninja3 - As far as I am aware, Miku only got interested in the mc when he told her liking history is fine and he wanted to genuinely help her. That's not love, not even close. Her whole life she was the odd one out and for the first time, someone took an interest in her as a person, even if it was just to help tutor her. It took her time to develop to the point that she was aware of her "love" for the mc. And it wasn't two damn days.
Shoujo has always been cringey with this kind of stuff. It's written in mind for little GIRLS ( like ages 8-15) that like fantasizing about handsome men, cute girls and drama. And, FYI girls tend to mature faster than boys so obviously they are interested in "romance" since that equates to "relationships" with the guy they find attractive.
Also, ironically, your whole point is shot down in this manga itself. Leading up to this point, the two main people were the brother and sister... And last time I checked, neither have any romantical interest in the other... Despite just meeting for the first time a few days prior.
And I'm gonna leave this as a friendly reminder: Use this https://www.grammarly.com/grammar-check for when you type stuff online. They also have a built-in browser app that makes it even easier, kinda like auto-correct on a smartphone. Believe me, you need it.