"Children" can be either an age category or a relationship category; I meant the relationship category. Sorry for not being clear about that.

SV, I think the simple way my Mom explained our similar questions about siblings (animal & human) was something about babies have a better chance of being deformed or not smart, when creatures from the same family mate. If they ask more, you can give them a quick explanation of how variety is needed to keep family genetics strong.

even if I was willing to tell him propaganda like that no way I'd want to answer all of the questions that would cause. He's an unusual 6 year old...I'd have to explain DNA at a higher level of science then I have in my brain to satisfy him.

Perhaps, John, but experience says, unlikely.
I had a nice fat crush on my much older handsome hippie artist second cousin when I was a little kid... and into being a bigger kid, come to think of it... Just a cool guy, from a great family. Just thinking of him still makes me smile. He grew up sexy, too... but *sigh* he didn't wait for me- he got married & divorced & married again...lived all over Europe. Very Crushworthy.

When I was 19, I spent a very date-like evening w/ that cousin's younger brother at a sea-side amusement park. He was in his mid-20s, & had just gotten back from working oil rigs to make money to finish college. I was pretty wild at that age, testing my sexual power, & I could tell my cousin was intrigued, if also bit protective. The energy was pretty juicy... enough for some clear flirting. We definitely had as much fun as when he & his brothers used to become scary growly monsters &, chase my brother & l, lol. (They let us have their old comic books, too).
We didn't act on it tho', & I'm more than OK w/ that.
Cousins aren't siblings, & second cousins is far removed enough not to be creepy, but still, it just would have been weird. Our families were really close, & the relationship was more like first close cousins.

I come from a pretty solid, close-knit Jewish family, . We're not conservative or repressive, but especially back then, there were few divorces, & our extended family was very stable. Still pretty stable, actually. So our blood ties & culture of family are very strong.. That usually tends to discourage acting on inter-family stuff. In addition, this was in the NYC metro area, & there were tons of extended family around who saw one another frequently & looked out for one another. Plus, certainly plenty of other people outside the clan around to choose from.
The idea of being sexual w/ immediate family is completely anti-sexual, disturbingly creepy & blecccch to me.

SV, other than the genetic reasons why siblings don't make the best mates (which isn't propaganda- there is real truth to that) there are lots of good reasons for incest taboos. You already cited the power issue. There's the obvious need to protect children & even teens. Much of that has to do w/ the physical effects, but it's also about a young person's right to grow up free to develop in their own time, emotionally & sexually.
And also, very importantly, even btwn siblings, the taboo promotes more stability in family groups. (Adult steps who are no longer living together is different). Sex can be binding, but we all know it also can cause a LOT of drama, & breakups happen. Bring that into already fraught , intense bloodties of family... & whoooo, what a powderkeg. Just turn on Jerry Springer...or think of people you know that have been thru it...