beta
Hajin was born first. The father told the mother that she had to get pregnant and do her job as a wife or she’s done for (presumably she’s been having an affair and he threatened the partner). Mother found out she was pregnant with the affair partners child when Hajin was around 4-5 and had the baby. Instead of letting people know that she cheated and had a bastard child, daddy dearest forced her to stay in England with the kid for a bit and then dumped the child onto the grandparents and mother came home without the newborn after father threatened to do something with that child and her partner. Her affair partner is most likely dead. Hajin being the legitimate son and the eldest probably took after papa in his cruel ways and bullied the illegitimate younger half-brother. Mama bear did not like that and started seeing hajin as daddy 2.0 and was NOT happy. So she hates hajin for what he represents. Her golden baby boy could do no wrong because that’s her lovers son. The whole “swap” thing is about their stories in England. The butler said that Hajin was the one who lived with the grandparents in England but it was actually the illegitimate brother that was in England.
I think the mother did something wrong in the past before Hajin was born and that the father is holding it over her head like a guillotine ready to come down at any moment that’s he decides he’s had enough of her insubordination (because he treats her like his lessor rather than his equal). He probably abused her mentally this whole time with the fact that he could easily kill the last remaining trace of her dearly departed lover whenever he wanted to and that it’s by his graciousness that the child lives.
If you think something is wrong with my theory or timeline, feel free to correct me but I’m pretty sure this is it.