I'm a 34 year old married guy. My wife is 32, and we have been married for 9 years. Recently, my wife's younger sister had been having some problems in her personal life, and wound up moving in with us. I wasn't really happy about it, because I like having my space and privacy, but I wasn't going to tell my wife no; as long as she wasn't here for too long. Kelly (my wife's sister) and I have always gotten along fine. Sometimes I think I get along with her better than my wife does, lol. She's 24, and she's almost like just a younger, slightly more curvy, party-girl version of my wife. 5'5, probably 175 lbs. or so. 36dd. She's thick, and likes to wear clothes that leave very little to the imagination. I don't know how many times there have been just around the house where I've had to make sure not to stare at her. And it's even worse when she goes out to have fun.

Anyway, just last Saturday, Kelly had gone out to a club with some friends. Both of our kids were spending the night at their grandma's, and my wife and I just spent the night at home, did some drinking, and had the same old boring sex before she went to sleep. Typical night without the kids.

I was laying in bed next to my sleeping wife when her sister texts me "can you come let me in? I can't find my keys." It's not surprising that she would try asking me first, because she knows it would anger my wife a lot more of she'd have asked her. It was almost 3:30 in the morning and she was coming home drunk.

I threw on some pants and ran downstairs to open the door and let her in. She was dressed to kill, as usual. A short, tight black dress that hugged her big round ass, and barely kept her tits in, and black heels. I'm sure I was staring but wasn't really worried. We made small talk for a little as she sat on the couch, taking off her shoes and then her bra. One of her tits popped out of her dress as she took off the bra, but I didn't say anything as she just went "oops get back in there" and tucked it back in.

She asked me if my wife and I had fun during our alone time, and I just told her "yeah, you know how that goes with her. Lots of fun." And we both laughed. I was still pretty buzzed, and I thought about staying up for a while to hang out, but figured it'd be best if I headed back to bed. I told her goodnight and headed back up.

I couldn't sleep. I think 30 minutes had passed, when I start to hear some moaning coming from downstairs. I got up, poked my head around the corner on the stairs, and there's Kelly, on the couch. Her phone in one hand as she stared at it, and her other hand down her panties, fingering herself. The moaning I heard was from the porn she was watching on her phone. I watched for a moment before shaking my head and turning around to head back upstairs. But before I went back in, I turned around to go look again. Kelly looks good, period. She had actually changed out of the dress she was wearing. Now she just had on this sphagetti strap tank top and some pretty small panties. There was plenty of light to reveal her ridiculous body.

I stood on the stairway and stared at her as she fingered herself. Eventually I started to stroke my hard-on. She was really going at it, and getting loud. I just kept jerking. But then she turned her head from her phone and looked right at me. She knew I was there. Only she didn't stop. She looked right at me and kept going, biting her lip and closing her eyes in the process, and then smiling. I froze for a moment. I was so turned on.

And then I went down the stairs. I slowly approached her as she kept pleasuring herself, and I stood right next to her, with my pants now down around my ass and my cock in my hand. I never had a shirt on to begin with. She just kept going, sometimes looking at me, sometimes closing her eyes. But she was definitely enjoying my presence.

I don't know how much time I let pass before I just decided to go for it. I took off my pants and boxers, and I moved onto the couch and in between her legs, as she opened them up to allow me in. I slid off her panties before fully getting into position, as she slid back a little bit to get more comfortable. I placed my cock right on the tip of her pussy and teased her. Then I wound up rubbing the tip of my cock on her clit until she was cumming. That didn't take long, as she had gotten herself most of the way there already. And when she came, and got pretty loud, I shoved my cock inside my wife's sister's pussy. I hadn't been with anyone besides my wife in over 12 years, and it felt great. She let out a loud moan and pulled me in, as she still shook from the orgasm she was having. I wound up pulling down the straps of her shirt and she helped get her arms out, and I pulled it down to expose those gorgeous big tits. I played with them as I started to fuck her slowly but hard. She moaned and closed her eyes, saying "fuck yeah. Fuck me, Tim. Fuck my little pussy." This was different for me already, lol, because my wife doesn't really talk dirty at all. I was into it.

I told her how fucking tight her pussy was, which was true. It gripped my cock much tighter than my wife's seems to. I actually had to fuck her pretty hard to go deeper and deeper inside her. We both talked dirty as I fucked her. And she'd ask me if she was better than her sister, my wife, and I was honest and told her "fuck yeah." She really seemed to like that.

I don't know how long I lasted, but I know it was a while. I was still buzzed, and I had already cum once earlier that night, so I was good to go, lol. When it was time for me to cum, being that I wasn't wearing a condom, I pulled out. But she told me "no... put it back in, please" in her best moan/whine voice. I told her "I'm going to cum." And she looked right into my eyes and said "put it back in." She didn't have to tell me twice. I shoved it back and and fucked her hard until I was unloading my cum inside her. She moaned and wrapped her legs around me. I kept my cock inside her and fingered her clit until she was cumming again.

I pulled out, and sat back. We stared at each other and smiled wide at me. Eventually she stood up and said "well fuck, that was fun." I just stared at her gorgeous body, as she stood in front of me wearing nothing but the tank top that was now crumpled up and down over her waist.

I wanted more. She looked great. I've always loved my wife's ass, but Kelly's is even bigger and more round. She noticed I was still staring at her, and she asked me "mmm... do you want to go again?"

I didn't even say anything. I just got up and stood in front of her, kissing and groping her hard. I turned her around and gripped her ass. She wound up getting down onto the floor and into doggy position. I fucked her like that until I came inside her again. She was LOUD. We both were. Looking back, it's amazing my wife didn't hear. It probably helped that she had been pretty drunk herself and must have been out cold. When we were done we just kind of laid in the living room floor, side by side, rubbing each other's bodies. Other than the obvious conversation about "year, we better not tell Michelle(my wife)", we were pretty quiet. I eventually wandered off to take a shower and to bed, and she went to sleep on the couch.

The next morning, when all of us were up, almost every time my wife would look away, Kelly would be smiling at me. She was still wearing the tank top I had fucked her in, and some pajama pants.

So far, my wife seems to have no clue, and neither one of us plans on saying anything. We have occasionally flirted and texted, but haven't done anything else yet. I think if we do get the opportunity again, though, we'll be fucking again.