Hmm... The relationship isn't as complicated as the situation. I really like how you focus on the surroundings. I hate humidity (it makes me feel heavy and tired) so I could definitely sympathesize with M's (don't want to misspell) feelings.
I can sort of imagine M feeling a bit confused and not completely himself. It's just all se sensory overload so it's nice to have something so simple as spaghetti to get grounded back on earth.

I'm really enjoying your story.
I agree on the part of incest not being normal, but I think it's feasible to write incest in a "normal" setting. Part of what makes up the genre is the complexity of the characters feelings. There are a couple of stories that focus on themes - like the glass house in Familial Glitch or Happiness.
It's possible to write a story like this in a normal setting but to be completely realistic, it would be one sided and it would be quite depressing. It would have to focus on the mind of the main character and the interactions between the main character and the sibling.

Interesting setting and character dynamic! I'm looking forward to reading more, and to seeing how things progress between the brothers. Personally, I like long chapters, but that's probably 'cause I'm greedy (the more the better : )).

Funny premise, also because all the others now saw them kiss know they are brothers... Even though the idea of the family not knowing he's coming not caring about him will have to be explained later, I guess, and is kind of hard to believe so I guess there must be a good explanation somewhere?
BTW, there is a moment in the chapter where you still call Heito Hirohi, which was kind of confusing until the AN..!

Honestly this isn't grabbing me like I Hate Summer. That story has a certain over the top vibe but it it also seems to grow naturally out of its surroundings, but this one feels more contrived. I think the names contribute to that feeling. But I'm sticking with it because I'm interested enough in the story and very interested in your writing.

As for the first/third person thing, I normally prefer a straight-up 3rd without switching perspective. You definitely have the skills to indicate other people's feelings from the outside, so I wouldn't switch for _that_ reason.

good story, but huge chapters! you could easily break it up into 3 or 4 chapters, maybe at the POV shifts so that it's easier to read.