A senior at my part-time job showed me a selfie of his fuck buddy, and it was my sister! My sister, who always nagged me for being useless and was super uptight and annoyingly serious, had become a slut after entering university and getting pampered by a guy. I was shocked to see the photo. The senior then invited me to join them for a threesome next time! Since I had been wanting to mess her up, I decided to participate. On that day, my sister tried to run away in surprise, but she was caught and ended up doing a threesome with him.
