Responses

exactly! before their parents even got married to each other, the boys' were already pretty much grown and didn't need much parental guidance. so it'd be strange for them to act as brothers because the way they met was technically more as friends. and their parents ended up divorcing shortly after later, so the step brother issue shouldn't even be a thing because they never saw each other like that in the first place. on top of that you're right, they are literal adults, they don't view each other as family, so why should we?
can't believe jxh ship haters' biggest issue about them...is the fact that they're step siblings. if that's the biggest problem to you, you're obviously not reading enough yaoi. since when was logic important in a fictional world? it's just a story, stop trying to make it make sense.