if you're gonna look and be more open to her words. you will understand why she even said that even myself if i'm the sister i will probably do that, yes mongryeong doesn't even want that but the sister was just looking at the future, she doesn't want him to suffer anymore because of the seoks, she knows that it's all her fault and second the reason why she said that because she's also the one who take care of mongryeong so she does have rights to say that it's for his best, but if we gonna look at the other point of view what she doesn't have is to stop their relationship that's why she's begging him to leave him alone she may look annoying but she suffered alot as well she became a mother in a young age. cursing her and saying bad stuff is a little bit harsh.