Haruka is a cancer on this story. Everything goes back to her, despite the fact that she's barely present. The trip is announced in chapter 13. For the next 30+ chapters, it, and Haruka, keep getting hyped up more and more. If there's a mystery, a secret, or some piece of missing information, chances are good that it goes back to Haruka.
I won't beat around the bush any further: Haruka and Miyu are lezzing it up on the trip. Haruka is a high-end lolita fashion model. Miyu is selling herself to Haruka so Haruka will lend (not give) her some lolita outfits, and give her money to make her own. The way the manga had built it up, you'd think they were doing travelling sex shows for rich lesbians, or doing gangbangs. She masturbates to herself (With a vibrator, and a full-body mirror) in these dresses at home, and keeps a physical album of the lesbian stuff she did on the trip in her closet, along with the lolita clothes. The album, and the fetish for herself in lolita clothes are a source of shame, and embarassment and tucked away for now. Almost all of this happens off screen, save for a single description of Miyu getting off to herself. The trip comes, and we're stuck with MC's POV back home, while the FMC goes off to do porn things away from the audience in a PORN novel. This "secret" is, by far, the second or third most vanilla thing to happen in this story, so I don't know why all the fucking mystery just to tell us after the fact.
Miyu is also scared of being caught in Lolita clothes in general because her parents and teacher told her she's seducing her classmates in the third grade, and was told to stop wearing them.
TL;DR: The trip was relatively vanilla lez stuff, happened off screen, and wasn't worth the buildup. Haruka is Miyu's benefactor on top of being friends, so she's extra protective/tolerant. Miyu gets off to herself in lolita outfits, so she sleeps with Haruka to get paid, and borrow outfits.
Miyu is not nearly this stoic in the Web Novel. This goes for her reaction to drinking cum for the first time too. Manga makes it look like she's used to all this, but she was shocked, and MC believed it was her first time ever due to the fact that she was so uncomposed despite offering herself like that.
Miyu has androphobia. She's ok with men in theory, but the moment they approach her, she's immediately turned off, no matter how attractive (explicitly stated that attractiveness is a turn-off, but this may just be rationalization). She's been confessed to by men and women. She thought she might be a lesbian, but she never found one she liked (Though she doesn't mind being touched by them in that way). Eventually got her harsh attitude after rejecting so many people. Miyu has never been with another man, and we personally saw the first time she drank cum. The reason she decided to start helping is because her brother, unlike all other men, didn't disgust her, despite the fact that she caught him masturbating, furiously, to a character with her name; she wanted to explore this new thing, and see what she'd be ok with.
The cum catching was done on a whim. She let the intrusive thoughts win once, and after realizing she's not bothered by it, and is actually only turned on by close relatives, she kept going (In for a penny, in for a pound). The lack of blowjob (among other things) was her trying to wrestle with the confused feelings. She's very much cognizant of the fact that it's her brother she'd doing this all with, and she's like to maintain some modicum of "Ew, no. I'm not going to suck my actual brother's dick!". Naturally, the acts escalate, sometimes by accident, sometimes deliberately, and she reluctantly loses her inhibitions.
TL;DR: Miyu is only into family, and hates attractive men. MC fits the bill to be her lover, and he's the only guy she's been with. Her boundaries are inconsistent because she's confused herself. She's bothered by her lack of resistance to doing more with MC, so she resists.
