My sister usually looks down on me for being a virgin, and today she was taking out her frustrations on me and attacking me for one reason or another. I happened to hold my unwashed, sweaty dick in front of her, and she sniffed it, grimaced, and was shocked, but the embarrassment of showing my penis to her made me get an erection instead. She was very reluctant, so when I tried to put my penis away, she seemed to get horny and pushed me down and climbed on top of me, so we had a lot of fun doing naughty things.