beta
If you read it carefully you'll realize that Haesoo was the more favored one between them but that doesn't mean he wasn't abused. He mentioned being deprived of water for diet (iirc) and being hit in their heads whenever the "lady" got the chance, meaning he wasn't exempted from the abuse. It's just that him and Haesol reacted differently from the trauma they received. Haesoo became quite "unhinged" and saw those memories as good times with his sister but Haesol, due to the trauma, had selective amnesia (if I'm not mistaken) and forgot about her childhood memories.
While it's true that he opened Haesol's deep wounds, I think we need to take consideration to Haesoo's experience as well and should not portray him as the bad one. It is always the crazy adults who should be blamed for ruining these children's lives.