Love the site, this is my first story! I come here often!
My sister is 20, and this happened just today... I was downstairs, and heard a moan coming from the bathroom and I slipped into the room next to it. I wondered who it was. Then I saw my sister's car parked outside. So I knew she was the only one home. As the moans got louder, my dick got harder. I heard the water come on in the tub, as she was using the faucet as her way of pleasure. It wasn't that much longer -- I heard a loud moan... then a grunt, and then the water went off. by this point, I was on my knees by the door.
She walked out quicker than I expected and found me there with my dick in my hands. I tried putting it away and acted like nothing happened, but it was too late. She came out in a towel, and she said she knew I was there. And she threatened to tell my parents if I didn't masturbate for her. In complete disbelief, I quickly agreed, and entered the bathroom. I got completely naked and began to jack off. I noticed she had also put her hand between her legs. Assuming she was going to have another orgasm, this just made me hornier, and around 20 seconds later she let one out... and so did I! That was the best orgasm of my life up until now and neither of us have spoken of this since.