My dad started dating a woman in Mexico when I was 13. The woman had a daughter who was one month older then I was. Her name was Anna, and we really clicked. Anna was beautiful and funny and loved to tease me. I had a huge crush. I'm a pale white boy and she looked like a beautiful indigenous native - think Rosario Dawson. Big lips, big round eyes, big smile. We would obviously spend a lot of time together because our parents were dating (and later got married). But before they married, my dad would bring me down on trips with him to Mexico where we would live with his girlfriend for weeks at a time. They had a separate bedroom in the backyard that was separated from the house, and so at night, Anna and I would be alone in the house.
It started with us just hanging out late at night, keeping one another company late at night. We got more comfortable with one another and would tease and push and tickle and flirt. And then one night we started lovingly stroking one another, and then kissing.
And during the next three years, when I would go with my dad to Mexico, Anna and I had a secret affair right under their noses.
We'd pretend to go to sleep, and our parents would say goodnight. After they would leave, I would cross the hall to her room and we would make out for hours on end. It was pure bliss. The secret aspect made it all the more thrilling.
I remember I couldn't wait for night to fall, and since we'd usually make out until 2 or 3am, I would be fucking brain dead during the day. And that's cause I had priorities. But we never had any sex, we just loved to make out for hours on end, and then during the day, in front of our parents, we would play it totally cool.
Our parents married and she's still my stepsister. We're both 29 now. And still, nobody knows but us. We're both in separate relationships now and probably only see each other once a year or so. But we get along great. Our teenage romance was a blast, and a memory I look back on fondly.
TL;DR My step sister and I had a secret affair and nobody ever knew about it.