This happened a few years back but it is crystal clear in my head. I have a little brother who is two years younger than me. I grew up with him and never had any sexual thoughts about him.
This changed one day when I came home and thought I was alone. I went upstairs and before I reached the top I stopped and gazed into my brothers room which could be clearly seen from the steps. What caught my eye was he was sitting in a chair completely naked. He must not have heard me cause he had not moved. He was 14 at the time, and I had not seen him naked since we were little. He was sitting low with his legs wide open, giving me a clear view of his dick which was rock hard. It looked a little smaller than mine, and he had a small amount of hair.
He was slowly moving his hand over it, I was in awe, I knew it was wrong to watch, but I couldn't look away. My dick was raging hard, for some reason this was turning me on beyond belief. I watched as his hand moved slow, touching his hard dick from top to bottom. Then all at once I saw a line of cum squirt out the tip. I almost came just from the sight. He sat a minute and walked to the bathroom. He shut the door and I quietly walked to the bathroom downstairs.
I pulled down my pants and jacked like crazy and in seconds shot cum all over the sink. I never told him that I saw him, I still masturbate to it today.