Hi, this happened just last week while is was staying at my nans house. My cousin is 18 also, short, average sized tits but good for her size. She caught me watching porn recently and didn't make a deal about it and the pushed me to try this.
We had to share a bed and I had been thinking about it all day. I was so horny and a small patch of pre cum had already started to appear on my undies. I thought that Maria was already fast asleep when I got in bed. I gently went under the covers and looked at her panties. They were a normal pair of cotton white undies but they were a little bit wet on the crotch! I thought, Jack pot!
I gently started to stroke her wet patch and smell it. Then, as I was stroking it, I heard a moan from above sheets. And gently called out, ' Maria?' she answered and I instantly came out from under the sheets and thought that I was in big trouble!
She then whispered, why'd you stop, with a little smile and I went back under the sheets and started stroking her mound again. After a while I pulled her soaking panties to the side and gently started to finger her. She was moaning louder now and told me to go faster. I did and started to feel her contract around my fingers, she started to moan louder and then she peed on my hand. I came up from under the covers and licked my fingers clean. Then she said she would have to return the favour.
I completly pulled off my undies exposing my erection. She took it in her hand and started it stroke. It felt SOO good! She spat on it a bit and kept pumping until I was almost about to explode. Then I said I was cumming, she pulled down her top exposing her perky tits and shot my cum all over them. She then licked it off and we went to sleep that night with my undies off and my penis touching her the whole night.
The next day we did some more but that's for another time.