We go to my wife's parents house , to visit . When we do , she and her mother go out for the day . Leaving me at home with her 13 year old sister . I am 21 . Her sister started to do things to get me horny . She flashed her panties . Sitting on a chair and opening her legs . She wear white cotton panties . Nothing fancy . She coud see what she was doing to me . No hiding my errection . All i could do was say i needed to go to the toilet , and wank . This was every time . She must have known what i was doing . Each week she did more . Lifting her dress to show me her panties and her bare stomach . Spreading her legs wide . Showing me her camel toe . Then on time when she flashed me . She was not wearing panties . Just a quick flash , giving me an erection . Having to to the toilet to wank . The following time , she spread her legs wide . showing me her pussy . I needed to wank . When i got up to go to the toilet . She still sat with her pussy on show . I gave in to temptation . I did not go to the toilet . I got down to lick her pussy . Expecting her to stop me . She let me do it . I took my pants off , and fucked her . She was so hot . It did not take long for me to cum . When i got dressed , i realised what i had done . Cumming in her pussy . I could have made her pregnant . After that i always took condoms with me . Breathing a great sigh of relief , when she had her period . Now as soon as they had gone out . We went to her room for sex . She started coming to stay with us . When my wife had gone out , we spent hours in bed having sex . We had sex for years , until she got married . As soon as she could , she went on contraception .