It seems my sister, during the afternoon, started an argue with her husband so she would have the excuse to leave and spend the night at our parents were I also live. And she did it, but around 12 AM (our parents are already asleep) I hear someone closing the front door, I go to the window and see my sister running for a guy that was waiting for her close to his car. They started to kiss and the guy grabbed my sister's ass just before they left in the car. I went downstairs and put and left the key in the door lock from inside, so she couldn't open it when she return. At 6 AM my phone rang, my sister was at the door asking me if I could go and open it before Dad and Mom wake up. I told sure but we have to find an agreement between us because I know too much. This happen the next night, she told her lover she couldn't meet him. I always knew my sister was a slut.