@PsychoTheRabbit Tbf, their parents got divorced, and that also can lead to lots of situations. Although Youngwon has Soyou and a family that loves him, he still has to live 24/7 with his dad, instead of the more close supportive system that Soyou has in her range (their mom and their aunt).
Sure, Soyou can feel suffocated because of the heart transplant and all that she lost within it, but I don't think it's fair to minimalize their parents' divorce's damage as it could have left emotional scars in each of them. I think it was mentioned earlier on that since the divorce happened, she hasn't really spent lots of time with her brother and she is worried a lot about him because "he doesn't act as he always did (around her)".