When I was quite young, my dad remarried and I received an older half sister (ten years difference).
When I was twelve and she was twenty-two I was allowed to go over and use the pool in her apartment complex. I would usually go over, swim and then she would either let me use the key to let myself back in or she would go up with me. If I went up alone, I would often look through her underwear and stuff.
One day when I went over, she had a friend over. We all went down to the pool as usual. When I wanted to go back up, they said they would be there in a minute or so, so I went up alone. I let myself in and went right to her underwear drawer. I was in the midst of masturbating when in they walked! I was mortified.
They laughed and said it was OK and everyone did it. Then they asked if I had ever seen a real naked girl? Of course I had not so they offered to show me, but only if I would masturbate more for them. I agreed. Cheryl (my sister) and Nadine (her friend) pulled off their bathing suits and laid down for me to see. They let me touch them and they touched me. I was pulling my fellow, but, let's face it, at twelve there was not much there. When I had cum they then proceeded to masturbate for me and said I could come over anytime I wanted and we could all use the pool.