idk if you even know what you're talking about when using the term "incest". inbreeding is typically taboo due to biological complications when siblings produce offspring/s. but not only are haesoo and joowon not blood related, they are also both male. i will assume that your only subject of reasoning is the social boycott of the issue which is totally overrated and pathetic. but other than that, there really isn't anything wrong with it.
You have the talent of being unable to separate fact from fiction. I- Ok. We're going over there
The crime of sexual relations or marriage between a man and a woman that are so closely linked by blood or affinity that such activity is prohibited by law.
Source: https://legal-dictionary.thefreedictionary.com/incests
Joowon and Haesoo's taboo is not consanguineous, but they were linked by an affinity (stepbrothers). This legal agreement (their parents' marriage) has ended for over a decade and they are only seen socially as siblings due to this past relationship. They don't see each other as such and their feelings will never leave them, their stepbrother past will always haunt them and this is the reason why they twisted their relationship even though they are no longer legally related. Their parents tried to re-impose the taboo on them a second time because they were so aware of their son's feelings. Joowon's father said it, there really is no longer a taboo that can prevent them from being together. Meaning ... They are no longer brothers.
Yes, it's weird that they fell in love when they were stepbrothers, but they didn't grow up together long enough to see each other in that way. They grew up seeing each other romantically and saw their relationship as their first love. It's a taboo based story, genius. Don't get moralistic when there are many more socially controversial plots than this one. If you don't like it, drop it.
"Affinity ordinarily terminates upon the Divorce or death of the blood relation through whom the relationship was formed." The Joowon and Haesoo's incest agenda is invalid even in real life, omg (▰˘◡˘▰)
https://legal-dictionary.thefreedictionary.com/incests
Their age definitely matters here too.
they were 16 when they met... almost legal adults. they were not impressionable little kids who will start calling each other brothers because they are told to (like if a mom tells their 7 year old, "this is your new brother now", the 7 year old will simply accept that this is their sibling without question). Haesoo and Joowon were far beyond that. They were old enough to see each other as merely strangers pushed together by their parents (unwillingly at that).... hence why they never saw each other as siblings, and were driven by their attraction to each other from the start.
lol if joowon and haesoo weren't step brothers. they would have been long lovey dovey and haesoo will not even give your boy taku the time of the day. and if you really think that people who likes joohae would support "incest" as per your narrative then i think you lack the common sense in the first place. idk touch some grass??
this still doesnt change the fact that haesoo and joowon were step brothers and caught feelings for each other WHILE THEY WERE STEP BROTHERS....