My younger sister was really getting creative on ways to let others see her pussy and making it appear accidental.  Our parents had told us that some family friends were going to be coming over this evening.  Our folks and this couple got together every so often to play cards.  The only thing that made it interesting was that they had twin daughters just a little younger than my sister.  She would sort of babysit them, even though they were close to the same age. I knew that my sister had something up her sleeve but all she would tell me was that she had to "fix" her babydoll nightie.  You might remember them.  They were very popular for young girls in the 70's.  It made me crazy when she wore hers.  The material was somewhat, but not totally, sheer.  So you could see through it, sort of. 

I knew that she had taken some sandpaper and rubbed the material on the top to thin the fabric.  And yes, it did make it more sheer.  She also had raised the hem up a few inches so the bottom of the hem was just barely longer than the tops of her bikini panties.  She was very aware that if she put the pantie part on somewhat carelessly that part of both ass cheeks would show or, sometimes she adjusted them so almost all of one ass cheek was naked.  This made me crazy with lust.  Sometimes one of her pussy lips would escape also.  I particularly remember that happening one time when we stayed with some family friends one summers.  Half of her pussy was ALWAYS out! I knew that she knew what she was doing because sometimes I'd slide my fingers into her slit and she was always very wet.  One time I did it in the kitchen.  We were behind the breakfast bar and our parents were on the other side eating dessert!  She was the horniest fifteen year old I knew.  Anyway, I wasn't at all sure what modifications she was going to make but couldn't wait to see the results.  She had spent an hour in her room working on it so I had high hopes. 

Our company showed up, we all ate dinner and then the adults went into the living room to play cards.  We kids went downstairs to watch TV.  I was very excited.  Bedtime couldn't come quick enough.  Usually all four of us got ready for bed because our parents would play cards till very late.  Shortly my mom hollered down the stairs for us kids to get ready for bed.  The two girls visiting would just be putting on their pj's but my sister and I would be showering.  Now bear in mind, the door to the bathroom was very much in sight of the living room and you could see almost all the way down the hall to the doors to our bedrooms.  My sister would shower first.  I waited in my bedroom with my hard cock.  I just could feel that sex with my sister would be happening shortly.   Minutes later there was a knock on my door.  I opened it and my sister stood there with a very short towel around her.  This was one of her favorite things, wearing this towel that showed a good 2 to 3 inches of her ass cheeks and pussy.  She told me that it was my turn in the shower.  I looked over her shoulder and saw our friends' dad looking at her.  Clearly he had to be seeing her naked ass.  I was glad my towel was in my room.  I used it to shield my erection from view.  

I went and took my shower.  I too, went to my room with just my towel wrapped around me.  Standing at my bed with my cock pointing straight in the air I had an idea.  I walked over to the wall and gave one small quiet tap.  I heard Beth's door open and just that quickly open my door.  She walked into the room and was followed by the two girls.  They couldn't see that I was totally naked until they were all the way in the room.  I acted shocked that they were seeing me naked but I didn't turn away.  I reached down and got my pj top and put it on first.  Slowly.  They were all getting a very good, very long look at my steel hard cock.  When I put my bottoms on I grabbed my cock and made a little show of placing it where I wanted it.  I really liked letting those girls see me handle my cock. 

While I was getting my pj's on, my sister was getting under the covers of my bed and told the two girls to do the same.  I then went on the side of the bed where my sister was, which was furthest from the door.  I kneeled on the floor by the bed and acted like I was interested in the story my sister was reading them.  Very shortly I reached under the covers and pulled her bikini bottoms down and then totally off.  I was already steel hard and this was really adding fuel to the fire.  I very slowly moved my hand up to her pussy.  It was dripping.  Very quietly, under my breath, I whispered "Tell me how many fingers" and then slide one finger into her tight wet cunt.  Not looking at me she mouthed the word "One".  

I slide my finger part way out and then slid two fingers into to her.  It was amazing how wet she was.  It was also amazing that she didn't seem to  skip a beat reading that story.  It did take a little longer for her to answer though.  "Two" she mouthed.  I then very slowly pulled partway out and added another finger, which I slid in really, really slowly.  She closed her eyes and paused her reading for a minute.  Then she opened her eyes and looked right at me.  This time she said very distinctly "Three."   She then slid down further in the bed and was laying almost flat.  The two girls had to do likewise if they wanted to see the pictures in the book they were reading. 

Then I very slowly slid my fingers out of her.  As I did so I made sure that the blanket that had previously covered her from her hips down slid down.  In fact it slid all the way down to mid-thigh totally exposing her pussy.  She acted like she had no idea that she was totally naked from the waist down.  I could see that the girls noticed right away.  But Beth just kept reading like she had no idea.  I inched my hand up and with those two girls watching I slowly worked four fingers deep up inside Beth.  I was in real danger of cumming just from the excitement.  Beth, still acting like she had no idea of what was going on spread her legs wide.  Her left leg actually draped on the one girl so she could give her pussy all the room it needed to accept her brothers hand.  I used my thumb to caress her clit.  The two girls were fascinated by all of this.  We sat there and listened to Beth read pretending we weren't watching an incredible sex show. 

Beth arched her hips up to meet my hand and gave a couple of quick bucking motions while she orgasmed on my hand.  The wet sucking noise being made while I fucked her with my hand were incredibly arousing.  I knew those two girls had to hear it.  I also knew that if anyone opened the door we were in real big trouble.  No way to lie your way out of this deal.  After Beth's breathing slowed down from her cum she looked at the girls and told them to go out to the kitchen and get themselves some dessert, she would be out there shortly.  The door no more than shut and I was on top of her.  I pulled her top up over her tits and slid my cock into her with one stroke. I fucked her hard for just a few strokes.  The prolonged delay of getting inside her cunt and the danger of having our parents just a few feet away from where we fucking really added to the intensity.  I pulled my cock out of her phenominally juicy pussy and my first jet of cum caught her right on her mouth and chin.  The rest of my cum went onto her tits and then a couple of spurts went on the top of her slit.   I rolled right off of her and told her she better get out to the kitchen and help the girls.

She jumped up, wiped her pussy with my bedsheet and put on her panties.  Out of my room she went.  Looking at her ass I could tell immediately what she had done to her nightie.  She had very carefully removed the liner.  You could easily see her ass through the sheer material.  I wondered what the front must look like.  I wiped off my deflating cock and got my pj's back on and arranged.  I went out to the kitchen to see what she looked like from the front.  I was very aroused but also concerned.  What if you could see her pussy and someone said something? 

There she was, dishing up ice cream and cake.  Acting like she didn't have a care in the world.  And yes, you could see her slit.  You could see the few black hairs she was just starting to grow.  And she served the adults their coffee and dessert too.  I watched from the kitchen door and could see from half the room away that her pussy was on total display.  When she moved just right you could see a spot and it was getting larger, from her juices.  No one said a word about it.  When she came back into the kitchen I told her what a great job she did.  She just smiled and said "Wait until next week!"  I didn't know what she was talking about but I knew it would be worth the wait!