When I was 15 my parents went away on holiday and left my sister, who is 3 years older than me, at home. While they were away my sister contracted earache, so she asked me to fill a hot water bottle for her. While doing this I managed to spill some boiling water over my hand. She told me to sit down for her to apply a bandage to the burnt area, as I sat down she leant forward and I could see right down the front of her nightie.
Not only did I get a view of her very large tits but I had a clear view of her dark bush. My penis became instantly hard and I had a job hiding it from my sister. She left the room and I took my now throbbing penis out of my pj's and with just a few strokes started ejaculating. When she came back into the room I'm sure she could see from my pj's that I had cum.