There is something that happened when I was 15 years old that still sticks in my head.

It was a Saturday around noon and I was get getting to go out and play. My mother tells me she needs to go out for the afternoon and I would need to look after my 10 year old sister. What made it worse is my sister had a friend over so I got stuck watching both of them. They went and did their thing and I just watched TV.

After hearing very little from them I decided to go check out what they were doing. They were in my sister's room, door wide open, her friend naked on her bed and my sister looking at her naked body. I asked what they were doing and they said they were playing Doctor and asked if I wanted to play. The friend was 10 years old but I was also a 15 year old boy so seeing any naked girl was of interest to me. Basically my sister and me were doctors examining her. My sister was telling her friend how to move and pose so we could examine her. Her friend did exactly what my sister said and literally allowed us to examine ever inch of her. We began doing a gynecology exam of her and she was very cooperative by letting us see everything. I saw the very intimate parts of her and was able to come in for a very close look both inside and out. The exam we were giving her also had my sister and me touch numerous areas of her body including the intimate areas. Once her exam was done her friend began to get dressed and told my sister it was her turn to be the patient.

My sister began to get undressed and was now up on the bed naked. I told her friend she would be the main doctor and I would assist. I had seen my sister naked before but not in the way we began to exam her. Just as with her friend she was being touched basically everywhere. The Gynecology exam was my first look at my sister in this way and I truly never thought I would ever see her this way. Watching my sister and her friend that day was turning out to be an unexpected surprise. My sister's exam was finished and they both told me it was my turn to be the patient. I knew they might ask me but didn't really know how to handle it. I was looking after them, was older and knew I shouldn't.
They asked me again and her friend told me she would help the patient get ready for the exam. Her friend came over and started to help me get undressed. I probably should have stopped her but I didn't and was soon laying naked on the bed with them examining me. I know my sister had only seen me naked a few times and her friend had no brothers. We both had single mothers so it was possible her friend may have been seeing her first penis and my sister was seeing mine for longer than she ever had before. I do remember both of them looking and paying a lot of attention to my penis. As with them they were touching me and had their hands on almost all areas on my body. They seemed to get excited when they touched my penis and it began to grow. Their exams only lasted about 15 minutes each but with mine they had it going on for almost an hour. It ended with them saying they should schedule another exam to check a few things again to make sure there was no problems. I got dressed and that was the end of playing doctor. To this day I still remember seeing them and the expressions they had when they saw me.