The story is very interesting and I enjoy the two main leads but I really hate how they handled the older sister and her rape.
She clearly isn't mentally stable as shown very early on with how obsessed she is with our main male lead as well as her extreme actions. Her parents, rather than raising her properly and helping her grow into a mature woman, spoiled her and treated her like a child. She still thinks like a kid would. Now that doesn't excuse her actions but she clearly can't tell right from wrong and I suspect just copies what her mother does. Her mother started bullying the MC and she just followed thinking it was okay because the whole illusion of 'parents are always right'.
I honestly thought early on that she had a chance to redeem herself. To realize that what she was doing was wrong and to ask for forgiveness, just like how a kid would when confronted for their action because mentally she is clearly a child. But that didn't happen. Obviously.
The way that they just dismiss her being RAPED and marrying her RAPIST is messed up. Even if she is a bad person, no one deserves such a fate. Being raped isn't only physical but mentally scarring and having her go through it only to drive her evilness and like conspiring with her rapist is messed up.