My sightly younger sister was way ahead of me in our teen years back in the early 60s. One day she called me into her bedroom to talk about something. She was sitting on the edge of her desk with her feet on her bed. I sat down on the bed. She was wearing a shorty nightgown with the matching short bottoms. She was moving her knees back and forth. It seems that the seam on her pj bottoms had split wide open. Each time she swung her knees to the sides I got a good view of her pussy which was about 18 inches in front of me at eye level.
This went on for some minutes and she suddenly noticed where my attention was. She looked down and saw the split seam and her face went as red as her labia. I asked her why her parts were so red and swollen and if she was infected or irritated down there. She looked at me, considered her answer and said, 'other things than infections or irritation can make that part of me red'. I said 'What other things?' She said 'Things that girls can do.' She paused for a moment and said 'You really don't know do you?' I shook my head. I asked 'Can you show me what you mean?'
She said 'I suppose so. But you are going to have to show me a few things too'. 'I suppose I could show you whatever you want if I know what it is.' 'I'm sure you do' she said. With that she flopped over onto her bed, raised her legs and slipped off the, PJ bottoms. Then she spread her legs wide and started rubbing. After a minute she started breathing louder. Her labia and clitoris started swelling up and getting redder and redder. All her parts got wetter and there was a sort of slurping sound as she rubbed. I watched closely and suddenly she started arching her back and making little noises and starting and stopping her rubbing. I said 'Neat'.
She said 'Now you know why and how I get all red down there. Now it's your turn'. 'What do you want to see?' I asked. 'You know how when you rub yourself some white stuff squirts out?' I said 'Yes' cautiously. 'That's what I want to see' she said. 'You can take off your pants and lie down on my bed'. I was already hard when I took my pants off. I stretched out on the bed and started jacking. In a couple of minutes my penis started throbbing and it got even harder. 'I'm about to squirt' I said. 'Oh good' she said. Suddenly the throbbing increased and gobs of semen started squirting out making a mess all around and my sister squeeled. 'Wow' she said. 'That's great. Can you do it again?' 'Not for a while' I said. 'It takes a little while, maybe half an hour until I can do it again.' 'That would be great' she said. I had no idea what she had in mind until that evening.