This happened about half an hour ago! I had gone round to my brother's house to rewire a telephone extension into his study to enable him to install broadband to his PC. Around 10:00pm his 16-year-old daughter, my niece, came in from an evening out with friends. She was wearing low-cut hipster jeans with a short vest top on. I said 'Hi' as she kissed me on the cheek, and I could smell a faint hint of perfume which aroused me somewhat. She was sorting out her school homework behind me, when I happened to glance round and caught her bending over. Her red thong was peeking well above her waistband, and I could see it was well tucked into the crack of her butt. By now my cock was beginning to stir in my sweatpants (I had no underwear on), so I shuffled my legs further under the PC desk so as not to make it obvious I was getting horny.
She asked if the internet connection on the PC was fixed now, and knelt beside me for a demonstration of how to use it. Her young and firm breasts which must be about a 36C were pressed against my upper arm as she got closer to see the PC screen better. This had the effect of making my cock stand to attention, and I had to re-adjust it with my opposite hand without her seeing. She kept pressing her breasts against my arm, and moving them up and down me, as she moved the mouse around. I was not sure if she realised the effect this was having on me, but she showed no signs of backing off, in fact she stood up and leant over the keyboard resulting in her breasts being about three inches from my face. I was able to see into her cleavage by looking sideways without moving my head. I could see one of her nipples, which looked very inviting, and I imagined sucking on it whilst stroking off.
She said 'Move back and let me sit down, so I can see what I am doing.' With that I skittled back the chair and she sat on my lap with her back to me. Her butt was pressing my rigid cock flat against my stomach, and I had a close-up view of that red thong. She rocked backwards and forwards slightly, as she used the mouse and keyboard. I had to lean forward to see round her, as I directed her on using the PC. I'm sure she could feel my hard cock pressing into her, but she gave no clue that it was so. She continued to rock to and fro, slowly jerking my cock as I watched her red thong riding up and down. I had to type some information on the keyboard, at which point she leant forward and her teenage breasts were resting on the back of my arms.
By now I was sure she was aware of what she was doing, but with the rest of her family elsewhere in the house, I decided against taking it further. 'It looks as though you know what you're doing so I'll be off now' I said. At which point she stood up, turned round and sat back down on my lap again. This time her pussy was now squeezed up against my cock, which she took great delight in rubbing up and down with the front of her jeans. 'Thanks Uncle, you're the best..' and she kissed me full on the lips. I slid my hands behind her and slowly stroked the top of her butt around the thong. 'You had better go now then' she said standing up, looking at my throbbing cock which had leaked a small amount of precum into my sweatpants. I left without saying goodbye to the rest of the family, and jumped straight into the car. I pulled over at the first secluded spot I could find. I whizzed the car seat back, freed my aching cock and jerked for all it was worth. Within seconds I had sent ropes of cum over the steering wheel, the windscreen and the floor. It was the most intense orgasm I had experienced for a long time, and all because of my naughty niece.