My twin sister and I grew up together in the same room until we were 14. While the bathing together ended at four or five, we still saw each other nude occasionally. Though we never said anything to our parents, we'd dress in the morning in front of each other and see each other nude. I would also take my underwear off when I went to sleep without my parents knowledge. My sister slept without panties. Mom worked at the high school cafeteria and left after us and got home before us. Dad left by 6:00am to catch the train and wasn't home till 6:00pm. We would occasionally spoke about our body parts but neither of us felt anything sexual towards each other.
School ended in our Freshman year of high school and we moved a few blocks away to a new house where we each had our own bedrooms. We were on one side of the house and shared a bathroom. Our parents on the other and they had their own bathroom. Mom had to get a full time job and was out herself at 6:00am every day and back about 5:30pm. We were to report to our Aunt's house about a block away once we got up and were ready for the day.
It was very normal when our parents were not there to show up in the bathroom nude, do our thing in front of each other (bathe, pee, brush teeth, etc) and be on our way. Nothing sexual. Funny we never did this when our parents were home like on weekends. We were both late bloomers and we even discussed it with each other thinking we might have something wrong with us. My sister started a little budding just before school started. I didn't start till about six months later.
Once school started we would go to our Aunt's after school (she has two little children). Next summer we started the same thing but after complaining our parents let us stay home alone but with no visitors. We could go out if texted or called about what we were doing.
One other note, my hard on points straight up, parallel to my abdomen. This will come into play.
Anyway, come the summer we're 16 years old. We both have part time jobs so we are in and out. We both have friends we see over the summer too. Neither of us is involved with a girlfriend or boyfriend. We're still doing our morning bathroom routine nude when our parents are out. One day a few days after my sister was at a sleep over, she comes in the bathroom. I'm brushing my teeth (first thing I do every morning). I have my normal raging morning wood. She walks up and says her friends at the sleep over were discussing their boyfriends dicks and they said they point straight out (perpendicular to their abdomen) when hard. She put her hand on my cock and pushed it down. It was both uncomfortable, border line painful but at the same time a turn on. She had only seen mine in the past and she asked me what was normal. I said I wasn't sure either but I said that in the porn we've both seen on line, most guys seem to point out and I'd never be able to do some of the positions they do. In fact I told her what she did was not too comfortable.
Now it is spring break and we're back in the bathroom after another sleep over. She walks in and says her friends were having the same discussion again and she said she'd seen her brother's cock while spying (she didn't want to admit our relationship) and it pointed straight up. As she said this, she put her hand around my cock and stroked up. I shuddered. She looked at me and realized what she did and asked if it felt good. I said yes. She asked if I wanted more; I said sure. She put her hand around my cock and stroked the bottom of my shaft but that was no good. I told her, do what she did before, so I took her hand and showed her. I swear, she didn't stroke more than four or five times and I came like never before. I shot streams of cum all over her, me and the bathroom floor.
After we cleaned up, I asked her if she could teach me how to 'please' her. She said yes and we returned to her bedroom and she showed me how to jill her. It was great rubbing her clit, fingering her and watching her wriggle in pleasure as she came.
When I get a chance, I'll write more about how our relationship has evolved and where we are now in college (summer break now but have some great plans for next year).