My wife and her sister always had a very close relationship and were very broad minded in their approach to sexual matters. They shared details of their sex lives and on a couple of occassions before marriage had shared a room to have sex with their boyfriends. So they had both seen each other getting fucked, giving blowjobs as had their boyfriends and in one instance my wife joined her sister and her sister's boyfriend and the boyfriend fucked them both. After we got married, my wife told me all this and i too was very open minded on most things. My wife's sister is not married but she has her regular fuck buddies. Recently we got together with her and a colleague of hers from work who she has been sleeping with on and off and went off on a short break together. We were in the same room lying around on the bed and talking and smoking. Then my SIL and her friend started kissing and me and my wife started doing the same..soon we were all undressed and fondling our partners. My wife started giving me a blowjob while her sister lay on her back as her friend started to fuck her. After some time my wife lay next to her sister and i started to go down on her. As i looked up I saw to my surprise that while fucking my SIL, her friend had leaned over and was now sucking my wife's breasts while my SIL looked on. Given that they had shared a guy earlier, they were cool and i too let them continue. This eventually led to my SIL's friend fucking my wife and her sister that day while I watched. I did not do anything with my SIL.