Su-a or whatever seems to have brown hair and eyes though and the mother has brown hair so I’m really not sure what you mean. The brown hair and eyes seems like it would be a very common coloration. I know a lot of people with that color hair and eyes.
Oh and additionally we don’t actually know how long it’s been since the mom died (at least I don’t recall a mention) so even if it is his child, it may not be cheating.
after reading ch.22-23, i don't think the dad cheated? (possible misconception could've been prevented if the author/artist had shown the wedding before sua's first entry into the house)
ml didn't come across as hostile towards his dad or stepmom (i assume he'd have been so if they were cheaters), just standoffish. i think his dad just remarried and had a kid while he was still in middle school... maybe he found it difficult to accept a stranger as a "replacement" of his mom and adjust to the new family dynamics? idk
the awkward tension between ml and his dad, however, could be bc they're not good at communicating their feelings to eachother and bc the dad is a typical "strong pillar" asian dad typa character (that's how it seemed to me)
Just by looking at the younger sister in chapter 22, the dad cheated on Chan-uk's mom. I'm calling it now. It's odd to have basically the same hair and eye color but a lighter shade on step child that you "didn't help to create". （￣へ￣）