Comment by take some of this on A Sibling's POV - Chapter 28

rather than trying to protect the peace he has between the two siblings, blondie should really focus on himself more. 

growing up without understanding familial love, he's obsessively clinging onto the relationship he'd created with them but in a wrong way entirely.

I still believe that he doesn't have any romantic affections for fmc, at least, untill he sorts out his own issues. when he states that whatever he's doing, he might make fmc’s brother mad— that motivation doesn't begin out of truly wanting her (in a romantic sense) but instead of wanting to forever be part of the family. 

everytime Ga-young is mentioned in his inner monologues, Kang-hyun will be brought up too. Blondie can't seem to separate them at all. (or maybe, I'm overthinking it and it's just because he's Kang-hyun’s friend first before he's Ga-young’s “older brother” but the 1st is more likely.)

Do-kyung should really focus on himself more than trying to protect his position. At least, that's how I see it.