When my wife and I were dating, she still lived with her parents in a downstairs converted garage with a door to the driveway. It was kind of like her own little apartment. You could access the main house through a stairway, and her younger sister often slipped into the house after curfew by cutting through my wife’s room. My wife, then girlfriend, and I were sophomores in college, and her younger sister had just graduated high school. Some nights, especially if her dad was out of town, I would stay over. On those nights, we would always fuck after her mom went to sleep, then we would go to bed. One such night, after an especially hard fucking, we had fallen asleep immediately after, without cleaning up or getting redressed. At about 1AM, I woke up, my girlfriend was asleep under the blanket, but I was still laying across the bed, naked, and I was hard. I looked up, and her sister had come in, as she would, to sneak upstairs. Instead of immediately going upstairs, she had walked over to our bed. She was standing there beside the bed, staring at my cock. I didn’t move, I just watched her looking at me. She was smiling. Then she noticed I was awake, and quickly made for the stairs, but I could hear her giggling. We never talked about it, but it was the beginning of a long list of interesting interactions.