Wife's sister

My wife was in hospital having problems with our third child she was carrying at the time. Her sister younger by two years came to care for our other two children while I worked. My wife would be in hospital until after the birth total of five weeks in all. By the second week of her hospital stay, my wife's parents took the other two children for a four-day trip. Leaving just me and my sister-in-law (SIL), we visited my wife in hospital each night, The third night my SIL accompanied me to a work-related event, at my wife suggestion and we enjoyed ourselves very much and overindulge drinking awaking naked together in bed next morning Sunday the children weren't due home till late in the day. We showered together, while still coming to terms with what had happened. When to visit my wife who saw us both looking troubled, asked how the event went. I answered it was very enjoyable and her sister just nodded. My wife knew what happens when I get carried away at such things. Quickly guessed what had happened due to us over drinking, as it was the cause of us having our second child. She wasn't upset, just smiled at us saying we must all have a threesome when she had recovered from having the baby. Which we did, my SIL didn't get pregnant. But became a willing addition to our love life after that on a semi-regular basis.