EVEN IF they are both biological child of their parents, they are not related by blood. Not even close as family. Chowon & Dojun were separated long before the kids arrive. They have no offspring and are not even in good terms (iirc). Soyi and Bin's family are not related, they could be family friends at most and not even directly but via Byul's family.
It's just awkward for Soyi and Bin because their parents are exes and were separated in bad terms.
Check back the diagram there is no line between Dojun-Bin or Chowon-Soyi. The line is between dojun and chowon only (exes).. Then dojun to soyi and chowon to bin.
Also, go read LIAI. There's ton of references from the orig story. It's also a good read and you'll appreciate Byul more when you read it.
Not really! Do you care for your mom or dad's exes? If you met your dad or mom's exes would you feel akward? Kids generally dont put that much weight to who their parents used to date, especially since it happened before they were even born. If your crush was the son of your dad's ex from the 80's would you care?
I mean if the ex ordered for one of your parents to be assaulted and emotionally abused your other parent, it would be awkward. If it where just a casual disagreement that ended amicably? Perhaps. But with such a messy end to the relationship and how the ex tried to break your parents up? Eeeeeehhhhhh no.
And how would you know that as their kid in high-school or middle school? Do you know all the relationship drama your parents went through? And why hold that over their kids? Their kids weren't even born when that happened so why bring them into the drama by telling them not to hang out or have innocent childhood crushes.

I thought they are cousins but they are half sisters