We had an interesting split in my family. The males were very liberal and open, but there was a clear divide between the sexes. I have two older brothers--one by three years and the other by six years. I shared a room with my brother who is three years older until I turned 14. It was nothing for us to be nude around one another--cramming into a bathroom, getting ready for school, running late so one gets in the shower while the other is brushing or shaving, changing to go swim or play basketball, cleaning up after chores, etc. None of that was sexual, it was just life. I grew up in a big family in a small house.
As I look back on the various experiences, even when sexual elements were present, it still wasn't about being sexual. For example, once we got cable, my brother and I would keep watch so that we could see some of the movies on Cinemax late at night. We found my dad's porn stash in the shed, and we would look at the magazines along with some cousins and friends. By this age, I guess starting at 11 for me, I was getting erections, growing hair, and cumming. It was not uncommon to just encounter other hard dicks...it just happened. My brother might be talking to a girl he liked. There's always a strange correlation between a guy making a dominant move on the basketball court and his dick being hard.
Eventually, my brother got a steady girlfriend around the time he turned 15, and they were always around each other, always so touchy feely. I was doig homework one night while my brother was on the phone with his girl, he called my name and when I turned around he had his cock in his hand. Not even fully hard, but it was thick and long and he was waving it around--like eight inches around and probably eight inches long. He thought nothing of it, at least from what I could tell, and it pissed me off--because I wasn't that big!
After that point, the stories and experiences get more explicitly sexual....the jerking off, the guidance, etc. but everything up to that point as just a part of the circumstances of our life. My dad was very free around the house. I always knew about my dad's side from very early on. It just wasn't a big thing. However, there was a clear line between the openness and freedom expressed between the sexes. For example, I've never seen any of my sisters naked. In underwear on occasion is we were running late or things were hurried for some reason, but never casually. This rule did not apply, however, if you were a younger sibling. For example, I have a sister 15 years older than I am who would give me baths....but I was like 3.