I think because as they kind of explained weirdly, since mc was born prematurely and given to the grandparents to raise, the mother was raising the Mc’s brother (white hair) so she grew a bond but not with MC as she probably assumed he’d die anyway so she’s really just toxic. But also she had white haired boy ripped away from her and traded for the mc after what like once he was a tween? so she probably felt he ruined everything and stole her older sons spot and now she can’t see him again. (But in honesty I’m not sure as it’s still confusing but that’s what I got from it, i don’t get why they didn’t just keep both sons, also if I’m missing anything or there’s something you know Please tell me :,0 )
The MC brother was the one who was born sick, and had to go to England to get better, while MC was born healthy. The infighting for rights in the MC family was brutal so the MC dad wanted a healthy son so MC mother gave birth to MC. She hates him because she was not happy giving birth to him and the fact they discarded her first son. She poured all her unhappiness onto the MC instead of the shitty family.

Why why does she hate her other son? !!!!
Truly doesn’t make any sense