Depending on how their created civilization viewed sexuality, there might not be this created taboo. There might be the natural aversion to incest, or there might not be. Since they were religious, there might be religious reasons to abstain. If the kids are that heavily religious that they are terrified of modern technology etc, it seems doubtful they would struggle too much with resisting temptation.
They didn't create a new civilization. The couple was born and raised in Russia. Their religion was a sort of Russian Orthodoxy. Whatever quirks they had, it's pretty fair to assume that they had an incest taboo (as do practically all human societies).
On the other hand, it's not obvious at all that resisting breaking that taboo would be as easy as not using modern technology. This was hinted in one of the book's reviews on Amazon:
> A spectra haunted this group, as well as other remote old believers - incest! Peskov never can definitely state this is the reason why the two brothers established separate dwellings six kilometers from the main housing unit, but certainly it is high on the speculation list.
I'd say that the taboo is now stronger than it has been. I'm half way through the excellent Montefiore book Jeresulem, the biography, and royal families and leader mentioned in it definitely didn't have strong taboos here. This may just be a ruling class thing I suppose.
Link to book review as the book is fascinating. http://www.guardian.co.uk/books/2011/jan/16/jerusalem-biogra...
That's just my take on the matter.