I recently discovered that my little sister has been, at the very least, sucking my dad's cock. I don't know the extent of it. I don't know how long it's been going on or if that's all they've done. I was spending the night at my parents house, in my old room. I couldn't really sleep and it was maybe 1am or so and I was going downstairs to try to find something to snack on. As I walked down the stairs I could hear the tv was on. Curious as to who was still awake, I poked my head around the turn in the stairs to try and see who it was. I couldn't see anyone, so I took another step until I had a view of the couch. And that's when I froze in my tracks, at the sight of my dad sitting there, pants around his ankles. My little sister was next to him on the couch, on her knees, leaning over and sucking his cock. She was still fully clothed, if you can call it that. Short shorts and a tank top. I just stood there and stared. I'm not sure how much time had passed before I caught myself just standing there staring, and now with a very stiff hard on. I stepped back to hide myself while still being able to watch. I watched the entire thing. When he came, he held her head down on his cock until he was done, and she came up gasping for air. She sat back upright and he stood up, pulled up his pants, and left the room. I went back to my room and jerked off, thinking about what I had just seen, wishing it was me instead. So now, it's been a couple days, and I've been back at my apartment. I haven't said anything to either of them yet. I'm very tempted to see if I can use this situation to my advantage. She looks good, there's no doubt about that. I do have a girlfriend, and our sex life is fine. But wow... that situation would be a completely different level.