But looking at it more objectively, Koi Kaze is a disappointment. What is it trying to do? It's not making you think, because the actions of the characters are never contextualised: why is a 30 y.o man acting like a rebellious teenager? Is it because he's lived somewhat emotionally sheltered? A trauma from his past? A personality repressed too long by society's crushing norms? We don't know, and nothing in his environment or the storytelling hints at anything but him being someone who tends to hide his feelings. Which is clearly not enough to justify acting on incestuous feelings. Harboring them is fine, no one can control one's feelings. But I expect an explanation as to why he acts on them.
You are putting expectation on this story that it's going to do something it has no interest in doing. It's not a psychological study of "why people develop incestuous feelings" or anything like that. Nor is it interested in being a deep analysis of incest. It's trying to be a literal slice-of-life story, of 2 ppl, who meet, fall in love, and its consequences.
I agree that art is not great, but I think it sets the mood well. The author definitely improved with his recent works, though even in this comic i liked the art style. And I've read shit ton of manga as well.
Also, I think that half of the things you complain about are explained in the story? He fell in love with her when he didn't know she was his sister. He did not develop the sisterly bond with her, as they separated when they were far too young, so they are effectively strangers. Most people do not commit incest not because they intelectually analyze the situation, but because for whatever reason they do not find their siblings attractive. There are theories that kids who are separated at an early age do not develop this aversion, which seems to be the basis for this story.
He is not acting like a rebelious tenager, he is acting like an adult that is frustrated with his personal life. As everythign else with the story, it does not seem to be a story of a guy that had a single traumatic event, but death by thousand small cuts, being slowly beaten into being old fashioned silent type guy.
We don't know, and nothing in his environment or the storytelling hints at anything but him being someone who tends to hide his feelings. Which is clearly not enough to justify acting on incestuous feelings. Harboring them is fine, no one can control one's feelings. But I expect an explanation as to why he acts on them.
I can get why you would not like it, but many people
The main couple lives in a world without friction
half of this story is about the expected friction and main characters trying to hide their desires from the world in fear of repercussions. The ending nearly literally spells out that they are going to live their life in constant fear of being found out by society. MC is likely to loose his job, or at least be strongly affected by his permanent "single" status, they have to hide their life from their parents and friends, something that obviously affects them. Personally I liked that there is no obvious "We all found out!" moment. Also, i might be wrong but the implication seems to be that the friend of MC is not treating him normally, she pretty much stops being his friend and becomes neutral work-colleague.
the story is structured linearly, without any surprise.
I can understand why you might not like it, personally I like this approach. It stays true to the genral mood of the manga, which is trying to create semi-realistic story, and it depicts it as it would happen, we know what will happen and we are left wondering if the author will do the thing, or if the story will change its course at the end (as it usually happens).
All in all, I sympathise with your disappointment, but I would say it has more to do with you just not liking this style of storytelling, then faults of the story.