This happened just the day I turned 20. I had started to explore my feminine side. My partner in crime was my friend. We were roughly the same age.
It had been some time since my friend and I were involved. We started out exploring each other's bodies as would anyone else. He also knew my inclination towards getting dressed. Times alone with each other were spent in playing house. I was the wife and he would be the husband. I would be the young wife waiting for my man to come home. Ready to receive him at the door. Make a cup of hot coffee for him as he relaxed. He would bring flowers and "surprise" me (We did this every week) and finally we would find time for a little romance and end up in bed.
Since we were of the same age and our parents knew one another, there was no restrictions on meeting up - Except for his sister. My guess is that she suspected what we were up to. Sometimes in fact he used to bring me her clothes to wear and I guess she had her suspicions. But as long as we were together she could never find any "proof". With time, we had learnt how to evade her very well. During the vacations is when she would really become a thorn in our side. We just could not evade her "Eye of Sauron". So as an alternative, we decided to meet up at my place. Parents never had a problem with that. They in fact thought we were studying together. So every now and then when he stayed back they weren't suspicious. We of course had some time at night - after they went to bed to play under the covers. Sometimes he would manage to smuggle his sister's clothes, so I could wear those for him. But most times... well we had to make do with our fertile imaginations. Most of the time, we would spend reading soft core magazines - especially the "Questions and Answers" section to see what other people "did" and how we could adopt those for our own purposes.
This incident happened on a day we had some guests around. Some of them stayed back overnight, so we had to make do with camp beds for everyone. He was around to help me out with my chores. Just that since we were amongst the last ones to wind down, we were asked to sleep on the roof. We had a few other people as well. He had very strategically put our bed around a corner on the roof. It was later I realized how strategic that was. But at that time I was just too tired to notice anything.