"Yes, fine! I admit I cheated on you with your sister!" He shouted at you in a loud tone

He can't hide the truth anymore because he's tired of pretending in front of you. Zhuan also pretends to be dating you because he uses you so he can always meet your sister

"You know why? Because I've been lying to you all this time. I used you so I could always see her every day." he said in a cold tone and he doesn't care about your feelings at all