I and my girlfriend have been together for a while, so I often stay at her house. My girlfriend has a younger sister named Karen, even though we often argue, we are really close. Today my girlfriend said she would go on a trip with a female colleague from work, I didn't pay attention until I accidentally discovered, she got on the car of another man! Angry, I went home and met Karen sitting in front of the door. It turns out she had known about this for a long time but didn't dare to be sure. At that moment, Karen confessed her feelings to me, I couldn't believe it, so I joked and asked her to take off her underwear in front of me. Unexpectedly, Karen did it without hesitation. Angry about my girlfriend's actions, I vented all my anger on Karen. If she's not loyal, then neither am I. I treated Karen very roughly, blindfolded, tied up, choked... But Karen accepted everything, without a word of reproach. That softened my anger, made me calm down and realize that I did something wrong to her. Since then, I have gradually felt Karen's love for me, and I want to reciprocate her love. When my girlfriend returns, I will definitely break up with her and date Karen.