I happened to see my mother teaching my brother...
I was a voracious reader by Grade 5, and I got 99% of my sex education from my extensive reading. I have a brother three years younger. He was a bit deficient and couldn't read well. My parents spent a lot of time reading to him and explaining things while I was left to my own devices.
A couple of years after I started jacking off, I walked naked out of the shower toward my room. I had to pass my brother's bedroom and the door was almost closed except for a crack. I heard Mom and him talking and I peered through the crack. Mom was reclining naked in this big beanbag chair. She had on just a sanitary pad held on by this white elastic belt thing that had clips front and back to attach the pad. This was 40 years ago before thongs were invented, but it looked like that. Pretty sexy.
Nudity was the standard in our house, my parents walked around naked and sometimes Dad had a boner. Saturday mornings, Dad would lie on top of her in bed for a real long time with the covers over them. Sometimes we would go in and interrupt and get 'shoo'd' away but they never closed the door. It took me 10 years or more to figure out they were having sex and he was IN my Mom's vagina!
I was used to seeing Mom's breasts. In retrospect they were probably a big C, maybe a D. She had a nice curve between her bellybutton and her bush, which was a light blonde-ish red. Anyway, my brother was asking Mom about why his penis got hard. He was only about 12 or 13, but he had a huge dick, bigger than mine, longer and thicker. I was getting hard just looking at them. I started touching myself.
He was half-lying and facing her. She started to stroke his cock and he put a leg over her and started thrusting his hips even though he was nowhere near her belly. She continued to stroke him and after a couple of minutes he cried out that he felt funny. Then, all of a sudden he showered Mom with ropes and ropes of his cum. I was so excited by this I came into my shower towel in a couple of big spurts.
Mom didn't look startled or anything that he came. Boy, was he amazed! She hugged him and he started to cry. She was comforting him, then she told him to go take a shower with her. I sneaked away to my room noiselessly. A few minutes later I heard them in the shower.
Nothing was ever said about this. About 10 or 11 years later my brother was married and had his first kid (of five).