My wife's youngest sister who is 7 or 8 years younger than my wife has very similar facial features with a very different body type (small busted, tight tiny ass). My wife has great C/D cups with a bit larger frame. I have always found my sister in law very attractive in a purely sexual way, she and my wife both know this as I've had conversations with both of them about it.
Years ago I needed to take injections several times per week, a task I hated due to the reactions my skin had. On the day I was due for my shot my wife was away on a work trip and I was drinking all day dreading the shot that had to be done. I called my wife and told her I didn't want do do my shots anymore because of the reactions, blah,blah, blah. Next thing I know my sister in law shows up saying it was time for my shot and she was there to do it as my wife had called her. Being as intoxicated as I was I thought why not, lets make it interesting. I told my sister in law I didn't want the shot, but maybe if she did it in my ass I wouldn't have the bad reactions I was getting in my stomach. She said "I don't care where it goes." That was all I needed to hear, I went to my bed, took my pants off (wasn't wearing underwear) laid on my stomach and yelled to her . She came in and asked if I was ready, I said yes, but I didn't want it. Before she could get the alcohol pad out, I rolled over on to my back with my cock fully exposed, I said " this would be easier for me if you gave me a blowjob first" she just looked at me,then my cock,then looked back at me for a few minutes I could tell she was seriously thinking about it but then said "you're too drunk." Not wanting to press the issue to far I rolled back over and let her inject me. It wasn't the only time she did my injections, and she usually got a good view of all of me when she did them.
After this I had further discussions with my wife about how I felt about her sister to see what she would say. While my wife has never given outright permission to fuck her sister, she has said that if I ever cheated on her (again) that she'd be more comfortable if it was with her sister, but she never wanted to know or hear about it. So I'm a little confused now. I have always had a flirty relationship with my wife's sister and I do know that if she ever tried to take it further than that, in a purely sexual manner I would, in an instant.