Well, it all depends upon where the author is planning on taking this.
After all, the reason she got involved with the previous chap was for revenge.
She had no feelings for him.
How does she really feel about Oppa?
Yes, her listed reason for pursuing him is to spite her sister.
But, she has known him for most of her life.
Is there any affection for him at any level?
Some people need an emotional attachment to have pleasure in sex.
She hasn't had that, so far, and found no joy in the act itself, just in getting revenge upon the girl who humiliated her.
So... there's the possibility the author plans to have her actually find herself enjoying it with Oppa.
Which would lead to all sorts of interesting reactions on her part.
Then again, she may just be a cast iron bitch.
It all depends upon where the author plans on taking this series.