One day during the summer when my parents were both at work, I was sitting in the living room watching TV when my older brother came home. He was 18 going on 19 and did landscaping work, and every day came home sweaty and dead tired. He told me he was going to go take a shower, and not long after he went upstairs I heard the water running. After about 15 minutes it shut off, then a couple after that I heard his bedroom door close.
A little later on I had a question for him and lightly knocked on his door, but he didn't answer. I cracked open his door and heard him snoring, but I got a shock when I walked in. He was laying on his belly in bed and his feet were hanging over the edge, but not only that, he was completely naked. I quietly closed the door and got a little closer, and that's when I noticed it, my brother had a big cock. It looked half hard and stuck out like a big log of meat. I just stood there for a minute watching it throb and twitch, but when I reached out to touch it, he stopped snoring and started moving around so I had to hide. We had an old house with big bedrooms and he had a recliner in the corner, so I ducked behind it before he rolled over. I half hid/half spied on him as he just laid there with it partially sticking up, but then he reached down and started squeezing and fondling it. In no time he was fully hard. It didn't look overly long, but was full and fat and his hand barely fit around it.
He started out easy making long, slow strokes, but soon picked up the pace and was full-on masturbating in front of me. Just watching it got me so wet, it felt like I had sat in a puddle. My toes were clenching as I touched my vagina through my shorts, and I could hear him start to talk to himself as he stroked. 'Oh yeah, take my cock, Sandy, take it all. Suck in all my cum!' Sandy was a girl that worked for the same company and was cute in a chubby doll kind of way, and I started imagining them naked together, him pushing that fat penis into her, making her moan and scream. He started bouncing up and down on his bed while he stroked. He grunted 'fuck me! Fuck me harder! MAKE ME CUM!' Then he spread his legs apart and let loose with a huge load of cum flying onto his chest, Then running onto his hand. He laid there for a minute catching his breath then got up and grabbed a dirty pair of underwear, wiping up all the cum and tossing it into the hamper. That big, dirty penis was still sticking out, but he shoved it into his shorts and threw on a shirt before he walked out. I heard him go outside, then ran back to my room and shoved my shorts and panties to my feet, rubbing myself and cumming in seconds, not once but twice.
The rest of the day I acted like nothing had happened and he had no clue, but I knew that we both had earth-shattering orgasms right after each other. I tried catching him again but never did, but I did masturbate non stop thinking about what I saw, and also imagining him and Sandy together.