First time I ever cummed

So the family and I went over to London for a couple of weeks as we had family over there and could have done with a break. Luckyily enough me and my older s**ter were sharing a bed. My younger sister was sleeping ina different room. Before this I only used to lick and kiss her feet in her sleep, but this was the first time i started having a thing for her ass. She was wearing satin pajama pants and a tshirt ass against my cock. I started feeling a sensation and started to get a bulge so I started grinding my hard cock against her satin ass then stopped to make sure she was sleeping. I thought I have to see what this really feels like. So I pulled down my boxers and just rested my cock on her fine ass and wow. It felt so good it was the first time treating my cock to my s***ers developing body. I grinding up and down whilst listening to her light moans in her sleep and lightly groping her ass till i just bursted out with my first ever load and couldnt believe it. I got up and ran to the bathroom to clean up and when I came back I switched the lamp on raised the duvet and fucking hell. My s***ers sexy ass covered in the biggest load I've ever gave her. I climbed back in facing the opposite direction and went back to sleep. Either she didnt notice or she thought she had a wet dream. Who knows. She never spoke of it.

What are your thoughts? Have you had any experiences like this?