Sister bliss

When i was seenteen my parents went out of town one week, and i was left alone with my sister sarah, she was 19 with a perfect figure, good sized perky breasts and long brown hair with blonde streaks died in, she was a real rocke chick.
My parents went out of town and on the first night they were away i was leaving the kitchen when my sister walked out of the bathroom wrapped only in a towel, the towel was small and barely covered her, she walked past quickly and i watched her perfect figure step up the stairs.
Later that night my sister and i had watched t.v together and made our way to our seperate rooms. i was feeling quite horny as i sat by my computer and so i loaded up a porno of some chick screaming as she was fucked by this huge guy. It loaded up and as i pressed play the sound of skin slapping against skin and the orgasmic moans of this woman echoed through the house, i quicky jumped up, my boxers at my ankles and turned down the volume. I sat down and continued to watch the pair on the screen pound against each other. Suddenly i heard my door open and my sister walked in to find me with my hard 7" cock in my hand and this porno plkaying on the computer screen, she looked at me and asked "what are you doing?" i answered with a sort of "ergh" sound.
Suddenly the expression on her face turned into a cheeky look of lust, she smiled