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Σ(っ°Д °;)っ Spoiler alert
They are not blood related. Seo in was adopted and in reality the never get along, his adoptive brother its a f** narcisist psychopath who always look foward to make seo in’s life a living hell. So they pretty much hate each other. The kiss was another fucked up thing his adoptive brother came up with to mess up with Seo in.

Boi I tried giving this a chance but why am I on ch 9 and come to find out that man he was kissing in higschool is his brother.