My girlfriend and I have been dating for a while, so I often stay over at her place. My girlfriend has a younger sister named Karen, and even though we often argue, we are really close. Today, my girlfriend said she was going on a trip with a female colleague from work, and I didn't pay much attention until I accidentally caught her getting into another man's car! Fuming with anger, I went home and met Karen sitting at the front door. It turns out she had known about this for a while but didn't dare to confirm it. Right at that moment, Karen confessed her feelings to me. I didn't believe her and joked, asking her to take off her panties and spread her legs in front of me. To my surprise, Karen did it without hesitation. Furious about my girlfriend's actions, I unleashed all my rage on Karen. If she wasn't loyal, then neither was I. I treated Karen extremely roughly, blindfolded her, tied her hands, choked her... But Karen accepted it all without a word of reproach. That softened my anger, making me calm down and realize that I had done something wrong to her. From that moment on, I gradually felt Karen's love for me, and I wanted to reciprocate her feelings. When my girlfriend returns, I will definitely break up with her and start dating Karen.