My sister in law
Neither my sister in law or I have ever told my wife that before my wife and I got married her sister and I had a one night stand. I was 28 and divorced and my sister in law was 35 and married. Her husband was out of town and she and I ended up at a party at a friends house. She got a little tipsy and I offered her a ride home. One thing led to another when we got to her house and we had s** until almost sun up. My sister in law has almost let it slip a time or two in conversations but we have managed to keep this secret for 27 years now.