Forty years ago, when she was my girlfriend's sister, I did the right thing and took my future sister- in-law hiking. It rained heavily and we got soaked so I suggested we light a fire under a rock overhang to dry ourselves. She was 15 at the time and suggested we should strip to our undies to dry our clothes better.
I had been keen to see her little body so why not I said, but we probably shouldnt tell anyone. At 15 she had really nice little tits, but the best arse ever and everything else perfect.
Needless to say looking at her in her bra and panties I was rock hard straight away. She couldnt help but notice because I'm fairly big and she just stared for a bit then asked me if she'd caused that. I said she had.
She said that since it was her fault, if I showed her what to do she would help me get rid of it.
I told her we should get naked and maybe we could help one another. We stripped our undies off and after having a good look at one another she came close enough for us to touch. I ran my hands over her breasts and then down between her legs and asked if it was okay. She said it felt better than when she did it to herself. I showed her how to grasp my cock and work it slowly back and forth. In no time we were in a great rythm together and both came very quickly.
I shot all over her stomach. It was the first time she had seen cum and she rubbed it around her tummy and tits.
We washed off in the rain and tried to dry our clothes, but had to stay there another two hours before the rain stopped. That gave us plenty of time to experiment with some more mutual masturbation and now, 40 years later, we still experiment sometimes.
She still looks fabulous and still has the best arse ever.