They have the same two parents. So, technically, when Hajin is in Uni as a first year, he was actually like 18 or 19 years old instead of 20... and his older brother, playing the role of his younger brother, is 20 pretending to be an 18 or 19 year old. But yes, they share mother and father. One takes after the father (MC) and the other the mother (the real Hajin), but both are hers and his - which is confusing to understand in the way it was written.

probably because i stopped reading and just recently picked it up again. but i'm confused, i just finished chapter 54. i don't understand, who is actually yoo hajin and baek hayeon, what's their relationship, i feel so dumb T_T