My DS is 8 and my DD is 4.
Sometimes they take my phone and make videos or take random pictures of each other and their toys. Not ever stopped them as I sometimes get lovely candid shots of each other. The other day I was looking in my gallery and found about 10 they took of each other's privates. I asked them about it and they really started giggling and it's obvious they did it because it was funny to them.
We lived in Germany for 2 years so they aren't perhaps as bothered about nudity as children in the UK would be and they still occasionally want to have baths together, mainly to have water fights.
I don't want to make an issue of it, I'm sure 20 years ago brothers and sisters looked at each other's bits too, it's just modern life that taking pictures is their way of approaching things, the DD sometimes gets him to photograph her back to see how her hair looks.
I've told them they shouldn't do this, it's not a nice joke. We do have conversations that no one should ask to look or touch them in the area covered by where their knickers go.I'm sure it's all innocent but still troubling. DS is at the stage where girls are yuck and complains if he catches me and DH exchanging chaste kisses in the kitchen