My sister and i comfortable naked

me and my sister shared a room growing up, and were very comfortable with each other naked. we did everything together, bathe together, dress together, sleep together. from an early age we were fascinated with each others bodies, especially what was different between us. my sister had a little slit between her legs, which we later learned was her vagina, and i had the opposite - dangly bits called the penis and testicles. we were always exploring each others genitals. as we got older, my sisters nipples began to grow puffy, and she giggled when i touched them. when we played together my penis would grow bigger and harder, to the fascination of my sister. we wondered if my penis would fit in her little slit. we stood up and i positioned my stiff penis at the entrance of my sisters pink slit. at first it was difficult, my sister had to stand with her legs apart to give my penis room. to our surprise, her vagina stretched to accomodate my growing penis, our genitals fit perfectly together! i tried to see how far my penis could go in, but her legs were in the way. i lifted my sisters left leg up and got a little bit further, but still could not get it all the way in. my sister suggested she lie on the bed and lift her legs up, and finally pushed all the way inside. she squeeled and asked if i had hurt her. "a little bit" she said, "but it feels good". not knowing what to do, i pulled my penis from her warm wet hole.
every night we slept naked together i put my penis in her vagina and we loved it. i left it inside her longer and discovered that pushing in and out of her vagina felt so good.
suddenly, one night my testicles started to feel tight and something spurted into my sister's vagina. it felt so good and we wondered what the warm sticky stuff was. from then on it happened every night.