I was going into 4th grade which meant my sister was about to go into 6th grade. We had been out by the pool all day. I think the first dare was for me to do a belly flop, but somehow we spent the whole day daring each other to do stuff. As we came in for dinner, I dared her to change in my room and run down the hall naked to get her clothes. She said, "Only if you do it to". So I went to her room, pulled off my suit and quickly took off for my room. We crossed in the hallway and I noticed her hairless crotch and the small bumps that were starting to form as b****, but neither of us slowed down and we got to our rooms uncaught. After dinner we went to the basement like we usually did to watch TV and I remembered I had learned something new. I dared her to "go commando". She didn't know what it was so I told her that it meant wearing her shorts without underwear. Again she said only if I did also. We took turns going to the bathroom and taking off our underwear and putting our shorts back on. As we watched TV I could tell both of us were nervous. Eventually she ran her hand over my p**** and said "everything okay in there". That simply touch brought me to full erection. "Not fair, I have more to show." "Not as much as you think, little brother." "Oh, yea, well I dare you to show me yours then." With that she slowly got up and pulled her shorts down. I was amazed to see close up what I saw in passing. I reached out to touch her, but she told me to take my shorts first. As I stood up and slid my shorts over my b****, she reached out and grabbed my p****. We spent the rest of the night touching and feeling each other. At one point my finger actually went between her lips and I felt her buck her hips against it. But after a little while she said she needed to go to bed and got up, grabbed her clothes and went to bed. Years later she admitted that she went upstairs and masturbated to an o***** for the first time.