When I was growing up my parents were cool with nudity. We were not nudists or anything, but they were cool with being seen in the nude as we ended up being.
When I was about ten and my sister was about twelve she asked if my penis ever became erect. I told her yes. She said the next time it was could she see it. No problem. I told her I could get it hard by rubbing the end a bit. I did and she thought it was neat. Sarah (her name) asked me if I liked boys or girls better. I said to play with boys of course but admitted to liking looking at naked girls. She admitted to the same thing. I asked her if she wanted to see dads pile of adult magazines. She said yes.
We went downstairs and I showed her a big pile in a corner. I told her my penis was getting hard. She wanted to see it. I said OK, but, she had to let me see her too. So, we both stripped off (our parents were out somewhere).
She had a bit of pubic hair (I liked that) and breast 'buds'. She let me CAREFULLY stick my finger in around the folds of her vagina and look at it and her breast close up. We agreed we would always let the other person look anytime they wanted. I rubbed my penis and showed her how I masturbated. I told her I called my penis my 'dinkery'.
Excuse me, I have to go now and have my way with myself thinking about those early days.