Several years ago my wife's 18 year old niece spent a week with us. The second night she was there my wife and I were making love. We didn't close our bedroom door because her niece had her door closed. We were in the doggie position when I saw her niece peeping around the corner. I didn't miss a beat and kept going. Knowing that she was looking at us made me super horny and we went at it until I had cum twice on the same hard. My wife acted like she didn't know anything about her niece watching which I don't think she did.
This happened every night for the rest of her stay. I could not get enough and my wife commented later about me being so horny. I did not dare tell her why.
Then one day about five years after the niece's visit I said something to her about it. At first she denied watching us but finally admitted that she did and said that she got so hot she had to masturbate after we finished. It took me about six months until I got her niece to masturbate in front of me. I think she did it simply because I agreed to masturbate in front of her. Now we not only masturbate with the other one watching, we masturbate each other.
I can't get her niece to take it any farther but I am working on it. But at least it's nice to have access to a young, very wet pussy to play with. I might also add that I enjoy sniffing the niece's dirty panties. Usually when we masturbate she will take her panties off and give to me to smell and lick while I jack off. Her niece's favorite thing is to catch some of my cum and rub my cum all over her pussy and especially her big clit. I have also used her pussy juice to lube up my dick. We have been doing this for about five years and my wife knows nothing about it.