I [37M] am in a happy marriage with my wife [41F] and 2 children. About 2 years ago she asked me if I could help inseminate her sister [37F]. It totally blew me away. Honestly, I always found her sister super attractive. She is younger, more extroverted, super fit since she is a yoga instructor. A little secret - I feel like in a different life if I met her sister first then we would be the ones married and my current wife would be the sister-in-law. Obviously I have never acted on this. My wife told me that she is worried about being childless given her advanced age, and just hasn't found an appropriate person to marry given the demands of her career. My wife said it wasn't a test and was practically begging me to accept, saying that I was the only man both of them could trust. She also said they didn't want to go through the whole process of IVF or artificial insemination, that natural insemination was way easier, cheaper, and with increased chance of fertility. After much discussion, I accepted.

I met with her sister, named "Sara" a month later during her fertile period. My wife kissed me as she dropped me off at her place, and told me to "have fun." I walked up to Sara's room, where she was already standing there, completely naked. After some awkward silence, she said "let me take off your clothes and let's just get to it." I told her that she was incredibly beautiful and that I've always had a slight crush on her, and that I might actually enjoy the sex. She let out a big smile. I asked if there were any ground rules, but she said there were none. She said we should both try to enjoy it, as this maximizes chances of conception. I enthusiastically agreed.

After some wonderful foreplay, we were ready for penetration. She rubbed some lubricant on my dick and slowly inserted it inside her. I could hear her let out a big moan. It was the best feeling in the world. We ended up having some amazing sex. It was so intimate as she caressed me, and we were just looking into each other's eyes the whole time and smiling. I told her how beautiful she was, and she said that my cock felt amazing. Finally I felt the urge to come and let her know, and she said "come in me" while moaning hard. It was so hot. I had one of the best orgasms of my life. We looked at each other in post-coital bliss. She looked as beautiful and glowing as ever. After about 1 hour I packed up my things and left after she gave me a short kiss. She thanked me profusely.

My wife met with me after and thanked me. She asked how it was and I said that it was fun and that I enjoyed it. She said she was glad it worked out so well. I ended up having sex with Sara 3 more times during her fertile period. I felt somewhat guilty that I secretly hoped she wouldn't get pregnant this time so we could try again later.

She ended up not getting pregnant immediately, so we ended up continuously trying for the next several months, upping it to about 4-5 times during her fertile period each time. Each time during the last sex of the cycle, it was super slow and passionate, not knowing if this would be the last time we would have sex. In the end, she finally got pregnant after about 5 months.

All in all, while I was sad the sex with her ended, it was a really special experience. It honestly felt like we were making love. My wife was super supportive of it and was glad we both enjoyed it, which made the experience even better. And soon my sister-in-law will have a beautiful baby who I will be able to call my niece, but in a more special way.

Even though we don't have sexual relations any more, it was nice to be able to experience that special connection with her and help her achieve her dream of motherhood. We have looked at each other with a special smile since.