The story is a bit disjointed, I'll give you that, and it does feel a little bit like it rushed towards a conclusion, but the progression of Mia's character is something that feels very natural.
As I've said before, Mia was a girl who felt undervalued by almost everyone. She sought validation in being rebellious and promiscuous, with particular emphasis on the latter. Sex felt fulfilling and made her feel like she was worth something, but it was ultimately a shallow feeling with no real substance to it. But with her brother, it's different. There's inherently a deeper connection there, but he also makes her feel her safe, makes her feel loved, because there are moments where he seems to show that he cares for her more than anyone else and only has eyes for her (which, however, isn't the case). Mia's never felt this before, and so naturally, she develops deeper feeling for him as she also thinks that the same is happening with him. However, the MC (being the dumb, horny guy he is) didn't pick up on Mia's feelings or didn't much less care because he was only interested in getting his rocks off wherever and whenever possible. But it took a huge moment like Mia finding out him and their Mum to shake him out of his complacency and realise that being with Mia meant more to him than he realised.
All this makes at least the basic premise of the story totally fine, even if it wasn't executed all that well. The slutty, provocative Mia was never meant to be who she was always going to be and she even acknowledges that in the letter. Mindless fuckfest games where the characters are all a bunch of one-dimensional, amoral assholes are far less successful than games that try to craft a decent story with well-rounded, three-dimensional characters and it's good to see the game going in this direction. It's even better to think that the game will (hopefully) more consistently be like this in the Ren'py remakes and present a far better version of this "stupid romantic story", as you would call it.
T_T
