I think it's because Cheol looks like the uncle and the dad fucking HATES the uncle for always being better than him aka uncle is rich af, smart, and like a ceo or something. So when he had a kid with the uncles face all he wanted to do was mess it up. The hatred was fueled even more when rich uncle started doing stuff for Cheol to make up for him not receiving love from his parents aka sponsoring him to go abroad, taking him places for just the two of them, and all around just being a dad. This pissed off the brother because he just thinks Cheol shouldn't be allowed to be happy at all so he steals everything he can away like the abroad opportunity, and toys uncle bought (I think). As far as I can remember that's why.
TLDR: Cheol look like uncle, dad hates uncle bc it reminds him he's a failure, dad beats Cheol.
Oohh, that’s a cool theory. Now that I think about it, that actually makes sense
Why does seme dad hate him so much tho? Is he even his real son? I find it weird when parents love the problematic son more than the smart one