
Not sure but in his (YW) flashbacks of the bedroom scene where SI suggested kissing— we can see that he thought about going along with it because he wanted to indulge in, what I think he said, were his “own selfish desires”. Now falling for, not sure exactly but we can see that he had some feelings be it lust based or crush.

Since YW never knew who SI’s crush was, it’s possible he misunderstood that his crush was his own stepbrother and YW here wasn’t disappointed because he liked him but rather it was this very thing that helped him figure out how much he liked him..that and the fact that SI kissed what (isn’t his bio brother but) still…his brother?!?

I have a feeling he was already in love but he never clocked it. He probably thought his feelings were just what you'd feel for any friend that you may occasionally indulge in some hanky-panky with. He must've really come to terms with his feels and errthing WHEN he saw the kiss, but by then he must've assumed it was too late
Love how everyone is bashing the bro so creatively lolll.
Btw, I'm super curious, at what point exactly did YW fall for SI ? He didn't seem jealous that SI likes "someone else" n didn't ask him. But it's clear that watching that goddamn kiss broke his heart so he must already be in love with SI atp.