I think casual nudity within families is very common, at least in this part of the world. I had a very conservative upbringing - never saw any of my family members naked. For a long time I thought this was the norm. But throughout the years I've had experiences and conversations that taught me otherwise.
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The first time was in the early years of high school. I stayed the weekend at a friends place. We spend the whole evening and early parts of the night playing video and board games. The next morning my friend and I were in the bathroom brushing our teeth, having a conversation about the X-Men, when his older sister walked in.
She had a short conversation with us, but I didn't say much because I was too distracted by her only wearing a towel.
I didn't know where to look, so I turned around. But they had a big mirror in the bathroom, so I could clearly see her turning on the shower, loosening her towel and hanging it on a hook, I saw her completely naked when she entered the shower cabin and she just casually took a shower behind us.
And I was like: "WTF! Dude! Your sister! She's naked in the shower behind us! What the hell?!!"
He didn't like it of course. He wanted us to get out of the bathroom. But what has been seen can't be unseen!
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For some families it's completely normal to see each other naked. They don't even bother locking the bathroom. It probably follows some sort of bell curve. On the left there are families like mine, who never see each other naked and on the right there families like that of my high school friend and my ex-girlfriend for whom it's completely normal to see each other naked (but they weren't nudists, nudist families would fall even further right, I guess). Most families fall in between. They don't see each other naked very often, but if it happens it's not a big deal.