Because she didn't open up, so no one really knew what she was going through..
I don't think it's right to blame it on that tho. Even if she didn't tell them everything it's still not her fault. A cry for help should be enough. In fact she called out for help multiple times but no one listened to her. Every child deserves protection from their parents, but it was denied to her so many times. She was never once treated like a daughter. Plus she was raised apart from her parents, it would be difficult to open up to them so quickly off the bat like that.
Anyone has a theory on how Sejin figured out it's Mirae he's been working and hanging out with all along? It feels…
i read a theory that he could tell by shaking both the twins' hands. Miji would have rough hands coz she did a lot of manual labour while Mirae was an office worker. Also Miji told him about his grandfather.
Plus she was raised apart from her parents, it would be difficult to open up to them so quickly off the bat like that.