MAYBE FOR BRINGING DOWN THE WALLS AROUND YOUR HEART!!!! I STILL THINK THAT NOAH IS LOST ROYALTY FROM A FAR AWAY KINGDOM(MENTIONED IN A PREVIOUS CHAPTER)
He's using the sister as leverage because the crown prince is willing to do anything for Euhnas. The only thing Euhnas wants is to run away with her sister but in order to do that the sister has to be freed first.
Are you sure??.. I specifically remember that young master who is "in love" with Enuha saying to the sister that they did not have any choice in the matter by getting married. Then he had sex with her and being able to do and wanting to do it because she looks similar to her younger sister.
So he's predicting her as he's downfall or weakness.