It's not that siblings are fucking each other, it's more that any 2 people are distantly related. I started using Geni, and it can connect me to virtually any other person who's done their geneology. I would imagine that for a small country like the Netherlands most natives are 10th cousins or closer.
Besides, the number of cousins in each generation increases exponentially and the risk in procreating decreases accordingly. I've heard that it's statistically perfectly safe to have kids with your 3rd cousin because there's so little shared genetic material.