dont tell me the guy he(ML) was kissing in the classroom was his brother /step brother....

I have bad news for you

Let me tell you - YES

Why would he kiss his brother??? HUH

this question will be answered in S2, but we can tell that the step-bro ain't a good guy. still, ML is a weirdo too for doing this erm.

We don't know the reason yet. But looks like this step bro had hots for seo in . But seo in had hots for MC.
So my guess is step bro initiated the kiss on his own , caught seo in off guard (?) or might have said something along the lines like 'i have someone I like, want to practice kissing' or some other bullshit.
This is just my conjecture , I'm also not a regular reader so I could be wrong.

I see, ty both (￣∇￣")

Do you remember which chapter shows he kissing his own bro?

tysm omg this helped so much