"Don't even think about it Loser!" laughed my sis-in-law as I was about to walk into the bathroom for my morning shower. I stopped and turned as she quickly pushed past me wearing just her nightie that hardly covered her ass. Ellen's sister, Karen, was hot. I know its probably wrong for a man to think like that about his sister-in-law but it was just fact. Ellen's parents had Karen later in life, so she was only half her age. She always used to joke that her sister was a mistake. Some mistake!! At 18, she'd just started Uni and was staying with us to save money till she could afford a place of her own. I didn't mind because she was a fox!! Karen had been just a small child when Ellen and I first got together, but now she was grown up, we were the best of friends. She often teased me by saying I was pretty cool "for an old guy!!" It was mid Saturday morning and I had slept in a bit as I had been working late the night before. Ellen had gone out shopping. Wearing only a bonds singlet and a pair of briefs, my semi-hard morning wood was on show. I had been stumbling towards the bathroom thinking of a hot shower and a good wank to start the day. So much for those plans... As my sis-in-law passed by me I noticed her eyes scanning my body and hover where my cock pressed against my underwear. "I'm showering first. You can wait your turn", she said as she pushed through the door and into the bathroom. "Besides by the look of it, you'll need to take care of things in here and I don't want to be waiting around for ever!!" she laughed giving my semi hard cock another look. It wasn't the first time she had made such comments and because of our close relationship I just laughed it off. Still half asleep I followed her into the bathroom and proceeded to the basin were I gave my face a splash with some cold water. Karen didn't object to me being there at all, just moved behind me to shut the door and turn on the heater lights before starting the taps on the shower. As I stood leaning against the bathroom sink looking at my slightly blood shot eyes in the mirror, I suddenly realised that my sis-in-law was pulling her nightie over her head!! My eyes were glued to the image of her nubile young body in the mirror as her pussy came into view followed by her pert little breasts. Her pussy was covered in soft blonde hair and looked like it hadn't been shaven in a few weeks, Her tits... wow! they stood firm and pointy without a hint of sag!! My eyes were feasting on her body until I realised she was staring right at me!! My arms went a little weak and I snapped back to reality as she said, "Had a good perv?" I span around and I apologised profusely but to my utter surprise she just said, "Don't be. I find it flattering that you would look at my body in that fashion, and by the looks of things you liked what you saw" she grinned. Her eyes again went to my crotch and as I looked down I realised I had come to full attention and was nearly poking out the top of my briefs!! With that, she turned and gave me a glimpse of her tight little arse before stepping into the shower. I stood and watched for a few seconds then quickly turned around again and grabbed my tooth brush to try and keep myself busy. I stood brushing my teeth but my eyes kept looking back to the image of Karen in the mirror. She was washing her hair with her eyes closed but was facing me, giving me a prime view of her sweet young body. I was still staring at her when she opened her eyes and caught me perving at her again. She didn't say anything this time, instead lathering up some soap in her hands and washing her body as she maintained eye contact with me. Her hands ran over her breasts and down over her tummy which was so flat and toned. My eyes were glued to her. She didn't try and turn around or cover herself up, just kept running her hands over her body. Her hands went down to her pussy and she rubbed it a little making my mouth drop open in awe. Suddenly she turned around and I thought maybe she had got annoyed so I went back back to brushing my teeth but then I heard her say "Could you bring over your razor, I need to use it" I spat out what tooth paste I had left in my mouth and slowly reached into the side draw to grab my razor. In the past I'd complained about her using my razor but I must admit I'd never thought she'd was shaving her pussy with it. I always assumed it had been her legs or armpits or something? By now the head of my straining cock was poking out the top of my briefs and I don't think I could of gotten any hornier. When I got to the shower door she opened it wide and said "You may as well get in here. You have seen it all and now I can see yours!!" I looked down at my cock sticking out over my briefs and without replying slowly removed them along with my singlet and stepped into the shower. I handed her my razor and closed the door behind me. I couldn't believe I was standing just centimeters from my naked sister-in-law. We shuffled around so I could get some of the water and she started to lather up her pussy again. As I stood staring at her, she proceeded to shave herself bare. It was all nearly too much for me, I nearly came just at the view!! When she finished, she stepped closer to me and reached behind me to put the razor on the little shelf. Her erect nipples came into contact with my chest and then I felt my cock touch her stomach. She stepped back and looked down at my hard cock. "Turn around and hand me the soap" she whispered in a voice that was only just audible. I did so and she started washing my shoulders and back. Slowly her hands moved down to my arse, then started moving round my hips and along the bottom of my stomach. Her tits were crushing against my back and her right hand grabbed hold of my shaft and started to rub. A groan escaped my lips followed by a moan from Karen as she started stroking my cock into a frenzy. I knew I wouldn't last long but also knew that I may never get a chance like this again. I reached behind me and placed my hand right between my sister-in-law's slightly parted legs. She moaned again as my fingers started to caress her pussy and then kissed my shoulder. It was all too much for me and I groaned that i was going to cum. She stroked faster as I erupted all over her hand and the shower floor. I kept caressing her pussy and as my orgasm subsided I felt her body tense up as she launched into one of her own. I kept stroking her clit until she pulled my hand from her quivering pussy. I turned to face her slowly and as I did she hugged me close. Whispering "Thank you" as our bodies locked together, my cock was still hard as steel and was pressing against her. "You're still so hard?" she said pushing herself against me. "Well you seem to have that effect" I whispered back letting my hands fall down her back to her arse. I puller her closer again and she moaned "Oh Fuck..." right in my ear. My horniness at an all time high I said, "Do you want to add "me" to the end of that statement?" She didn't say anything and I thought maybe I'd pushed it a bit too far until she slowly lent back against the wall, lifted one leg up and planted her foot on the wall behind us. "Me" she said with the sexiest grin i had ever seen. She grabbed my cock and slid it straight inside her soaking pussy. She let out a long sweet moan saying "Oh god yes, fuck me!!" I started pumping slowly in and out of her and she leaned back flexing her hips towards me. I was in heaven and we kept fucking until the hot water had run out. I quickly turned the taps off and we jumped out of the shower. I lifted her arse up onto the bathroom sink and as I stood between her open thighs, she looked right into my eyes and said "Come on old man, fuck your little sister-in-law's pussy..." I needed no further invitation and started thrusting deep and hard into her. "Harder" she said breathlessly, "Fuck my pussy harder." She was now nearly screaming. I held her hips and started fucking her as hard as I could until her body went into mad spasms of ecstasy which in turn sent me over the edge and I unloaded deep inside her. I collapsed against her, our chests heaving in time. I let out a little laugh and she asked me, "What are you laughing at?" I told her "Oh, nothing really..." What I'd been thinking about was lying in bed tonight and telling Ellen, "I've been a naughty boy..."