At the start of last Summer my wife and I decided on a trial seperation. We have been married for 12 years but things weren't good and we were constantly fighting. We don't have any children so she went to stay with her younger sister who I hate and who hates me back equally. I reckon she is responsible for a lot of the problems we were having as she has a real talent of being able to poison my wife's mind. About 2 weeks after we split I was woken by a phone call at about 1.00 on a Saturday night. It was Sylvia my wife's sister and it sounded like she was in a nightclub or at a party. She sounded drunk and told me that I was just a loser and that my wife was snogging some other guy as we spoke. I heard her giggling with someone else and then she hung up. About 20 minutes later the phone rang again and I could hear the same background noise but nobody speaking. The phone went dead and rang again a couple of minutes later. It was Sylvia again, but it sounded like she had accidentally called me and I could just hear people talking but not what they were saying. I did recognise my wife's voice and heard a couple of men speaking before the phone went dead again. At that stage I was wide awake and really curious about what was going on. The phone didn't ring after that but when I woke the next morning there was a text on my mobile from Sylvia. It said "I didn't know Rena (my wife) was such a screamer". Rena is very vocal during sex and so I took this as Sylvia telling me my wife had been fucked. I wasn't as angry as I probably should have been and actually got a bit of a kick thinking of my wife being screwed by another man. I decided to freak Sylvia out a bit and so sent her a reply saying "I hope he was black and hung like a horse". A couple of hours later I got a reply saying "White but very big i'm told". I didn't reply and went out that evening for a few pints with some mates. At about 9.00 i got a text from Sylvia, "She's seeing him again tonight". I replied "Why should I care". "She's your wife and she's fucking someone else and you don't care?" was her reply. I still don't know why because I have never felt any attraction to Sylvia, despite the fact that she is pretty good looking and has a great figure, but I replied "I don't care because I want to fuck you slut". I instantly regretted it as soon as I pressed send and waited for the fallout. Instead her reply surprised me, "what makes you think I would want to fuck you idiot". This sounded a bit like an opening and because of the booze I went with it. "Because I'll make you scream like Rena you whore". I got no reply and continued drinking with my mates. At about a quarter to 1 I got a phone call. It was Sylvia and she sounded pissed. "Guess what your wife is doing" she asked. "Fucking someone" I replied. "Give the man a cigar", she sneered back at me. "Seems like she gets to do all the fucking, what's wrong with you then Sylvia, not able to pull" I sneered back. "Fuck off you prick, I can pull anyone I want", she said. "Well you are only 10 minutes away, why don't you come over here and prove it slut", I said and hung up. I don't know what was making me act like that, but at that moment I couldn't think of anything I would rather do than bury my cock up Sylvia's cunt. Unsurprisingly she didn't come round. A week or so later I spoke to Rena. She admited that she had fucked someone else and told me that she was really sorry and that she loved me and wanted to come back. We got back together and were happy and things were good. A month ago I met Sylvia in a local pub. She was with a few friends and I was with a couple of my mates. She taunted me by asking whether I was capable of satisfying Rena after she had experienced a proper fuck. I told her she wouldn't know what a proper fuck was. At that moment she just went silent and stared at me. I knew she wasn't trying to think of something nasty to say by the look in her eyes. "Why don't you show me then" she said. I knew what she meant but was so stunned I replied "show you what?". "Do you want to fuck me or what you ididot", she said. "Christ, yes" I replied. She took me by the hand and brought me outside the pub. She pulled me to her and gave me the most incredible kiss. We snogged for a few minutes and then she said "let's go back to my place". We did and within minutes of getting in to her flat my prick was buried inside her. The first fuck only lasted a couple of minutes, but the second is something that will remain in my mind forever. I have never enjoyed being with a woman more than that time. I would love to have spent the whole night with her but had to get back to Rena. I have met up with her 3 times since and the sex is incredible. It is very strange fucking your wife's sister. They are so similar in many ways, but so different in others. Strangely enough my sex life with Rena has been great ever since, though I do feel guilty. Especially when she insisted I fuck her as soon as I got home the other evening. I hadn't washed my cock since fucking Sylvia earlier in the day, so my dick still had Sylvia's dried pussy juice on it as I fucked Rena.