I can recommend have your sister-in-laws staying with you.
It was about 6 years ago when my wife and I had her sister living with us. C was 24 and I was 28, I had known her long before I had got married but never asked her out and had always wanted to sleep with her.
C was going to get married but was dumped 2 weeks before, so we said she could live with us until she got back on her feet. (she had to sell her house). C had hired a sun bed and my wife was under it and I was laying naked on the floor talking to my wife, I heard something from the bedroom door and stood up as C walked in naked. We both just stood there and looked at each others bodies, she was thinner than my wife and had smaller boobs (34b)(wife 38c)and had a ckeekie little smile on her face and she blow me a kiss and walked out. My wife knew but didn't see her blow the kiss.
The next day, my wife was out shopping and I was at work and C was at home. I was only working half day and got home at about 12 noon. As I walked up the stairs to get changed, I could hear moaning and a buzzing sound comming from C bed room. The door was slightly open, so I peeked in. C was naked on the bed using a vibe on her clit as she tweaked her left nipple. I couldn't beleave what I was seeing. I took out my cock and started wanking off very slowly. After about 5mins I just wanted to be on that bed with her, so I called to C 'are you ok in there, it sounds like you are crying' and she replied ' I know what you are doing behind that door, come in and join me' my fantasy had come true so I walk in. I sat on the bed with my cock in my hand. C asked if I would sit between her legs facing her so I did. We were so close that my cock was almost touching her clit. I asked if we could do each other and C said yes. She took hold of my cock and started to pull very slowly and very hard I had the vibe and was pushing it in and out of her. C ask if I would tell her when I was going to cum so I did. she moved until her tits and face where close to my cock and started to pull me off faster, I came on her chin and boobs and she let out a loud moan. We both fell in a heap on the bed.
We must have fell asleep because I woke up with a start when the front door closed, it was my wife. I crept in to the bathroom and C just pretended she was asleep. My wife came up stairs and started talking to C. I walk out of the bathroom and sat on the bed as if nothing had happened.
C and I never talk about it but when ever sex is talked about we both just smile at each other. We are still very close and C is married. I think something may happen again due to the way we talk when my wife is not home.