For me, this is just an honest appraisal of a childhood. I don't think these actions would be particularly rare. Admitting to them later in life is rare.
Because of the huge taboo about incest and sex with minors most people would never mention this. Ever. But I wonder how unsurprising this might really be.
If I look at any mammal, there aren't any rules to sex. They do what they can get away with. Bonobos screw anything that moves, dogs use sex as a sign of domination, chimps wank each other off, and none of them are worried about the age of the 'victim'. Pop a hamster in a cage with his 5 sisters and he's not going to be deterred.
I don't find sibling incest very eyebrow raising. Similar ages, close proximity, both as ugly or as good looking as one another, and there is an argument that you are attracted to people with similar genetic traits. Siblings separated at birth and meeting much later in life and falling in love is something that makes headlines but happens way more than you might expect. Yes, you aren't supposed to have sex with your siblings in a 'healthy society' but actually the odds of a deformed baby are incredibly low. Sex with a cousin only raises the odds of a genetic defect by 4% over the normal population. Sex with a sibling 7%-31% over normal. But considering how low the risk is normally, its a tiny risk. Looking back to our cave people ancestors, you'd likely find a lot of it. Small nomadic populations, no rules, no law.
Regarding abuse, who would remember someone peering into their vagina aged one? A doctor might do that if you are ill.
I think the backlash is that people expect each other to be citizens, not creatures. To abide all rules regardless of curiosity, sexual impulse or sense of mischief. Religion could help prevent such things. If a child feels God is watching them all the time, they may refrain from doing something naughty. Without God, ... your odds of nothing like this ever happening when the kids are alone diminishes.
Think past the law, think past the morality, think past the rules and expectation of society. Kids aren't born with those concepts. Imagine 2 cave children left alone. Would they ever do this?
I'd offer sibling incest isn't a natural no-no. More of a preference should other suitable mates not be around. And abuse is something that pervades the mammalian world. I wouldn't call a 7 year old that did this disgusting. It doesn't shock me as unnatural or unexplained.