What if Jaewon did something to his twin?

Possible. Very possible. Bro is a certified psycho. He probably wanted all the attention and love and hated sharing. If he addressed that he detested seo in because he isn't his true twin, I would think he cared for his dead twin. But he doesn't seem to acknowledge his dead twin at all and just hates seo in for existing in their family. And from the way the mom is acting, jaewon must have done this before. Perhaps to his twin. Heck she might have some idea that Jaewon killed his twin but refuses to acknowledge that it happened to protect her plastic family.