I've seen a lot of fan threads that the author of Mashle was not expecting and put off by all the Ames brother incest fanwork that popped up after Rayne's true intentions for Finn (you know that whole part where it's revealed Finn is the most important to him), which is why he didn't draw out a full reconciliation scene, just implied it with Rayne's rabbits jumping on Finn and almost why he responded with Rayne being extreme person in QnA instead of saying it was because Finn was attached. (P1)
I'm assuming you're talking about Q54 from the fanbook Q&A? I think the rationale behind the theory is because the questions descended in the order of the characters the author talked about, and for Milo's question he used a submitted question that talked about Rayne instead (even Olore's question was related to his magic). Maybe they think the author used up Milo's question because he wanted to clear things up with Rayne instead.
I don't think that's true, personally, since there are other "useless" questions in the Q&A if the author wanted to dedicate one answer to say Rayne is the attached one and not getting rid of Milo's question. I can't read japanese and don't keep up with the author's statements so JP fans might know something I don't, but I don't think the author held back the reconciliation scene because he's disgusted at the JP fans' incest obsession for the following reasons:
This is no secret. If you were there during episode 1 airing of the elusive samurai you probably already knew—they sexualized the hell out of the 8 years old main character and even responded to western fans' disbelief with some crazy justifications. Mashle is still a manga that ran in Jump, and no way their editors wouldn't advise Komoto to make attractive characters that the fanbase can love (exhibit a: lemon's whole presence). This is why I can never take someone seriously when they used official anime arts or collab/merch arts to say 'this ship is canon just look at this official art!'. I love my gay ships, but using fujo baits to justify fanon ship is just falling into a similar pit as the antis tbh
But anyway, I'm sure even if the author doesn't know, his marketing team would've told him about the incestous fans. The author is japanese so I'm sure he ran into worse things just by the virtue of growing up in the digital age alone 🥲. He might or might not approve of the incest fanbase of the Ames bro, but I don't think he'd actively withhold a reconciliation scene just because of that.
2. Finn isn't attached to Rayne
Finn has always been the victim of shota and feminization, an unfortunate fact that I've always had to keep in mind whenever I need to block a passionate artist. The anime also drew into this, but if you notice, even until the end Komoto never 'feminized' Finn. This might or might not had to do with him not being able to draw women, but if you compared Meliadoul to Finn the difference is clear.
Meliadoul acts manly, and the author made sure to portray her that way as well, but you can still tell that she's female from how her face is drawn and the shape of her curves (I mean this in the most literal way and not lewd). As for Finn, he's noticeably less 'manly' than his peers, but you can still tell that he's a guy from the way the author drew the lines that make up Finn.
Along with this, the author doesn't portray Finn as attached to Rayne—if anything he's 'normal' about a distancing familial relation. The only time Finn thought about Rayne is during Mash's home visit when he wasn't sure what he should do to keep days like this going. And that's fair, since if a teen isn't sure what they should do they'd obviously think about what an 'adult' role model would do in their place. The author had every opportunity pre Ames backstory to add in Finn missing Rayne (like how he sprinkled Wirth wanting to catch up to Orter in his backstory) because atp we already knew they weren't close. If anything, Rayne had always been depicted as the obsessed one, which leads to my third point.
3. Rayne isn't the type to make a big deal out of feelings
This is basically it. Finn already aired out his grievances with Rayne during the delisaster fight and fought alongside his older brother during vs. Doom. Rayne already told him what he thinks and acknowledged his younger bro's abilities. I'm not sure what kind of 'full reconciliation scenes' fans are looking for, but I think the one we already have fits the best for both their characters. Finn is reserved and selfless, so if you put just him and Rayne in the same room for privacy he'd never be able to say what he wants. Rayne won't change his mind until he come face to face with proof that Finn had grown up without him noticing.
But if they're referring to the fact we never got them talking one-to-one after the whole battle then I guess? But the series had never been about them so adding a full reconciliation at the end is kinda awkward, and Mashle isn't the only series to end 1-2 chapters after the final fight. I think putting it in the fanbook serves the same purpose tbh
As for why there's such a large incest fanbase...I'm actually not sure? There's a theory running around that people are attracted to those who offer them unconditional love just for being who they are, which siblings technically falls into that category. It's why a lot of the times it turns out they're not even related, but the series I follow don't have an emphasis on romance so I can't speak for all of them.
It's said that people who are in love with the idea of incest are usually only children, which fits some asian families since they can't afford more than one child...but there's no official source on this and just my theory lol :))
