I am married to a very sexy 29 year old lady, sex is regular and fulfilling and we often experiment but have no desire to stray. That was until her sister came down to stay for a week. My wife and her sister are of a very similar build, medium tits, perk arses, long fair hair with my wife being a few years younger. I had always fancied her sister but loyalty and opportunity stopped anything happening. I came home from work one night and the house was relatively quiet, i assumed my sister-in-law had taken our two kids out to give us some time to ourselves. I needed a piss so walked into the bathroom undoing my fly as i entered. I was greeted by a lovely arse as my "wife" leant over the sink to look in the mirror. I was semi erect immediately and purred "hmmm lovely as ever" and slipped my hand into her arse crack and down into a surprisingly moist pussy slit. "what you been doing?" i teased but no reply. My cock was now hard and taking the chance of a quick shag before the kids came back slid my cock to her pussy hole. She parted her legs and I slipped in easily and began to push into her to meet her willing thrusts back. I had pumped about 10 times when a strange feeling of something not being right came over me....my "wife" was just that little bit noisier today (which excited me) and pushing harder than i could remember. I carried on but the uncertainty grew in me and I slowed down. "Don't stop you bastard, I ain't been fucked in a month and i'm close to cumming". Shit, it's not my wife, it's her fucking sister !!! Panic set in as she turned round. She could see the worry in my face, "shhh, don't worry i'm not going to say anything" and she began to massage my cock. I tried to resist but she simply bent down and slid her lips around my now rapidly hardening member. She then lifts her leg onto the rim of the bath giving me a great view of her very wet, gaping fanny. Her fingers began to explore her clit, my wife would never masturbate in front of me, and then slowly one, then two fingers disappeared up her fanny hole. I was wanking furiously at this great show but there was a fuck to finish. once again i offered my cock to her hole.."make sure you pull out when i tell you" she commanded. I was a little confused but what the hell. I slid in and fucked her for about 2 mins when she screamed for me to get out. As I pulled my cock out it was covered in a gush of female cum, something i had never seen before. I was amazed and without realizing my cock was now being wanked furiously but i came some 30 secs later in great spurts. We exchanged small talk and agreed that my wife must never find out. "No" she smiled. That night in bed my wife reached into my boxers and whilst massaging my balls casually asked me if I enjoyed fucking her sister ?! I tried to deny it but i had been set up. Both of them had discussed me fucking my sister-in-law and as it turns out my wife and her sister had experimented sexual techniques on each other when they were younger, something they had carried on whenever they met, right up to about 10 mins before i got in, hence the wet fanny. That night I had the sexual treat of a lifetime. I had had threesomes before but two sisters.....fucking fantastic. I had never seen my wife so explicit and our own sex life blossomed from then on. That was nearly 3 years ago and things have carried on whenever we visit each other which is approx 6 times a year. Funny prior to this I only used to see her sister at Xmas reunions...........