When I was like 12 or 13, me and my cousin used to play house every time we stayed at our grandma's house, we were about the same age. Grandma lived in a really old house where the bathroom had no door, just blinder curtains covering where the door was suppose to be. One day, grandma had to go to work and we were left alone for almost the entire day, she went to take a shower and I started peaking through the curtain to see her naked. I had a massive boner and right then and there I realized that my dick wasn't only for peeing.
She came out of the shower and we kept playing house. She noticed the big bulge in my pants and asked me what was in my pants that. I told her that my dick was hard and she wanted to see it. I showed it to her, she grabbed it and we started kissing. I grabbed her boobs and started sucking on them, then we laid down in bed and started trying to fuck, neither of us knew what we were doing and it happened more than once. I remember one day, I finally knew how to put it inside her and fucked her until I came, we thought she was pregnant so we stopped doing it and never told anyone. 30 years later, today nobody in the family knows our little secret.