I understand why you're mad, but things like this happen in the real world.
Her step-sister has three people on her side but Chong-Sa doesn't have anyone! She's all alone! (3:1)
second of all:
Her father is out of the house most of the time too. And when a man loves a woman, they'll do whatever they say. And since her step-mother is a really good actress, it's even more convincing to him.
You're still young, so there's a lot that you don't understand about in the adult world. But ask your mother!
That's why people say, the most pitiful child is one without a mother.