One summer during college I was staying at my brother's house in the guest room over their garage. My brother and his wife decided to take advantage of my presence by taking a vacation and leaving me in charge of watching over my 16yo niece for a week. I'm not into the incest scene (not passing judgement here but it's just not my thing) so I didn't have any plans in mind of a sexual nature. My niece and I have always gotten along well and I was just looking forward to a relaxing week.
The first few days of being chaperone went smoothly but then on Friday afternoon my niece approached me and asked if she could take the car to go out with her friends that night. She had just gotten her license and I suspected there might be drinking involved so I told her I'd prefer she have her friends come over and they party at home. That night she had 3 other girls over and I bought pizzas and rented them a movie. I stayed up in my room over the garage for the most part and checked on them occasionally. By 11pm they had retreated to my niece's bedroom and there was music and laughing and a few trips back and forth to the kitchen. So I went into the family room to watch a movie on cable (didn't have cable in my garage room). I closed the double doors to the family room so I wouldn't be in their way at all and settled in.
Around 1am it was quiet upstairs and I assumed they had gone to sleep. My movie ended and I surfed around until I found some low budget skin flick like they always show late at night on Showtime. I had the volume really low, turned off the lights, and was laying on the couch with one hand inside my shorts stroking an ever growing hard cock. Suddenly the door opened and one of my nieces friends (I'll call her Lisa) walked in wearing only a long t-shirt that went almost to her knees. I quick pulled my hand out of my shorts and moved to hide my obvious erection. She said the others had all crashed but she couldn't sleep and wondered if it was okay if she watched tv with me for awhile. I said that was fine so she sat down on the floor a few feet in front of me and off to one side.
The movie was showing a scene where a young housewife in a bikini was trying to seduce the pool maintenance guy. I knew they would be showing skin in a matter of minutes so I told Lisa she was free to change channels to something else. She checked out a few other stations but after a couple minutes went back to the sexy movie and set the remote aside without saying a word about it. It was a good thing that she was facing the tv and couldn't see the erection I was getting just a few feet behind her. I turned over so I could hide it as best I could yet still let my fingers trace the outline through my shorts. Lisa grabbed a few pillows from a chair and layed down on her stomach, propping herself up on her elbows and her legs pointing back toward me. Her t-shirt covered halfway down her thighs and try as I might I couldn't see much from the angle I had.
The couple in the movie were undressing each other now and things were heating up on the screen. I was going half crazy because I wanted to take my cock out so bad. Together we watched in silence as the couple started to make love. Although they never show anything like penetration on these cables movies, there was plenty of naked flesh and not much left to the imagination. Lisa seemed to be squirming around very subtly and I swore her hips were moving back and forth a little. I couldn't stand it anymore so I got up to lay the other direction on the couch where my view would be right up between her legs. Just as I got up she glanced back and saw the obvious tent in the front of my shorts. She didn't say a word as I layed back down the other way but she repositioned herself a few times over the next 10 or 15 minutes until her t-shirt had ridden up to the bottom of her ass cheeks. Each time she moved her legs seemed to move apart a few inches. I think she enjoyed the power she had over me as she teased me bit by bit. Now I could see right up between her legs all the way to where her panties were bunched up between the firm cheeks of her sweet little ass.
I wasn't even watching the movie anymore. I had my eyes fixed on her ass and sometimes as the glow from the tv brightened I thought I could detect a wet spot on her panties. I needed relief and I needed it bad so I took a chance and slipped my hand back inside my shorts. My cock had been oozing pre-cum for some time and I quickly had it nice and slippery as I stroked myself. I wanted to take it out so bad but this was just too close to home and I knew things could just as easily turn into a disaster if I went too far.
That's when I noticed for the first time that I could see the reflection of one of her legs in the glass doors at the bottom of the entertainment center. I froze for a minute as it occurred to me that from her position on the floor she could probably see my full body strecthed out on the couch behind her. I pulled the front of my shorts down far enough that a few inches of my cock sprang out. The minute I did that she bent her legs up and started swinging her feet back and forth which caused her t-shirt to slide halfway up her ass. I pulled the rest of my cock out and her legs spread apart even further as if to encourage me. Now I could see the outsides of her pussy lips with her panties pressed tight in between. She was rocking back and forth making the fabric of her panties slide along her slit and her movements got more and more obvious. My cock and balls were fully exposed and I pulled my shirt off so I could use it to catch my cum. I saw her arm move underneath her and her hips raised up an inch or two. She did a good job of hiding what her hand was doing but after a few more minutes I saw her leg muscles start to tense. That pushed me over the edge and I shot a huge puddle of cum onto my shirt as both of our bodies went rigid.
Her body relaxed and her hand came out from under her. I would have loved to have licked those fingers off right then. I quickly stuffed my cock back in my shorts and wadded up my cum-soaked shirt. We layed there for a few more minutes pretending nothing had happened. Then she got up and said that she thought she could sleep now, and said good night. I said we'd both be sleeping soundly tonight and she gave a small laugh as she left the room.
After she left I got down and layed on the floor where she had been. Sure enough, I could see the entire couch in the dim reflection of the glass doors. By the time I locked up the house and got back to my room over the garage I had to masturbate again as I replayed in my mind what had just happened.