A few years ago, before my wife and I (24 at the time) had any kids, we took a weekend trip to go visit one of her good friends in a town about two hours out. It was just going to be the two of us until her younger sister (20 at the time) heard who we were going to visit and asked to come along, since they were also very good friends when they were younger, and my wife agreed.
We checked in to our hotel when we arrived and went out to meet her friend and hung out the rest of the day. We got back to the hotel a little late and a little tipsy, wife and I climbed into our bed and sis in law into hers. About 10 minutes later we heard her sleeping heavily so we figured she had crashed out, and my wife started grinding on me for a little action. We started going at it quietly, but every time I thought I heard the breathing stop from the bed next to us I looked over my shoulder to make sure she was still asleep. We finally finished up and went to bed.
The next night there was no awkward looks or silence so I assumed everything went off without a hitch. We head out for another day of fun. By the time we got back to the hotel my wife hit the bed like a ton of bricks. I was still thinking of the night before and was feeling kind of horny, so after I looked over to check that my sis in law was asleep, I started tugging one out under the sheets.
After a minute or two I began to get lost in the moment, so it scared the hell out of me when I looked over and saw my sis in law laying in her bed, eyes wide open and staring at me jerking off. I froze, not sure what to do here. We stared at each other for what felt like ages, but then she did something unexpected. Without breaking eye contact, she licked her lips suggestively, then raised one of her legs under the covers, enough to lift the covers so that I could see she had her hand down the front of her shorts, slowly rubbing her pussy.
I felt a rush of blood through my body and my dick instantly went full-on rock hard. I started slowly stroking again, still keeping eye contact with her as we both started picking up the pace working ourselves. She seemed to keep flicking her eyes to my dick which was still under the covers, so I lifted one leg enough to slide the sheets up and expose myself to her. She smiled at this and responded by reaching down and sliding her shorts off, so that I could see the cute pink sheer thong she was wearing underneath, and went back to working her pussy as we watched each other. Our stroking getting faster and faster.
I could see her breathing getting harder and her eyes looking at me more lustily, flicking between my eyes and cock. She then pulled her panties aside and I had a clear view of her beautifully shaved pussy, and her fingers working furiously between the lips. The moonlight entering the room glistened off the juices she had worked up on herself. I was in a fever pitch. I couldn't take my eyes off it and was stroking madly. Until I heard her whisper, under lustful breath 'watch me cum'. I looked up, and she slid one hand down her tank and slid one of her perfect breasts out, and began tweaking the nipple, as she worked her fingers in and out of her pussy wildly. Her eyes squinted and she turned her head and bit her pillow, I could her hear muffled orgasm moans. That put me over the top and I shot my load on top of myself. When she had finished she turned back to me, and we kept eye contact for a bit as we kept slowly rubbing ourselves. After a bit she covered back up and I grabbed a tissue to clean up. We smiled at each other and rolled over to sleep.
The next morning, we had a free minute where we talked without my wife around. She told me she had woken up the first night and saw my wife and I having sex. She had started to finger herself while she watched but kept pretending to sleep when I looked over, so never really got off. The next night she was hoping it would happen again so she could finish herself this time, but thought that what we did instead was much better. She didn't see it as cheating, since we were only masturbating and not having sex. Her only regret being that she didn't get to actually see me shoot my load, since she had her head turned in the pillow. I told her I was up to try it again if she was. That night, our goal was to get my wife tired enough that nothing would wake her and we would try again.
The plan was a success, and my wife went to sleep very quickly when she got in the room. This time, we were ready. I got in bed, and she slid into bed, wearing a semi sheer purple tank top, and sexy matching sheer thong. It started the same way, lifting the covers so we could see each other working ourselves. Again sliding her panties to the side so I had a clear shot of her working her pussy. But after a bit of this something new happened, she slid over in bed, and patted the bed for me to come next to her. Another rush hit me, and before I knew it I was sliding in bed next to her. She threw the cover back over us both, and whispered in my ear, in a voice that drove me wild 'I'm not going to miss it this time'. With that, she took hold of my cock and started stroking it herself. I didn't skip a beat, and reached over with my hand and started fingering her in return. I could feel her breathing heavily on my neck, her eyes staring down under the covers as she stroked me faster and faster. I told her I was ready to come, and she broke away from that spot and leaned in under the covers, face less than an inch from the tip of my cock and pulled me over so I was laying on my side. I could feel her breath on my dick and it sent me over the top again. I shot a hot load that landed all over her chest and shirt.
She stroked me slowly for a few seconds more, then flipped back over to face me so we were laying next to each other again. She looked down at her cum soaked shirt and said with a smile 'well I guess this isn't any good anymore' and took it right off. I could see her beautiful breasts in full view just inches from my face. She then leaned in and whispered 'my turn'. She pulled my arms around her in a hug and turned over so her back was to me. Then took one of my hands and placed it under her panties, and took my other and placed it on her right breast. I went to work and started rubbing her clit and tweaking her nipple. She responded with really heavy breathing and light moaning, and arching her back so her ass started rubbing up against my cock between us, instantly making me hard again. As she was getting closer to cumming she reached one hand back and started working my dick behind her back. She turned her head and told me she wanted us to come together. I kept working her clit while she worked me and ground harder into me. She again turned her head to bite her pillow and I felt her body tense up and shudder. I responded by again reaching orgasm, only this time all over her lower back and ass. After we finished, she took my hand and I put it in her mouth and she sucked her juices off my fingers. She then slid her panties down and used them to wipe the cum I left on her back, again smiling at me and saying 'guess these aren't any good either'. We stayed in bed a bit longer and I held her naked body against me, I don't think my dick ever went soft for the rest of the night until I had to move back to my bed.
She must have woken up early and changed because by the time we woke up she was in a new set of pajamas. We finished up the last day of our trip without mentioning anything and went back home. We have talked about it a few times and once in a while shared mutual sessions over the phone or quickly in person when we can steal time. She has since gone on to marry and have her own family so we haven't met up in years. But the last time I asked her what she wanted when we go to visit on her birthday, she told me 'A blast from the past'.