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Genetic sexual attraction seems to be true from the little bit of reading I've done, it's because you're genetically programmed to like people who share traits with you. Without knowing each other through childhood the mechanisms to prevent incest never kick in.

 

These two shouldn't be having kids though that's for sure, way too closer relatives gives way too high a chance of messing up their baby. Also while personally I find the idea of the relationship disturbing, if they didn't have kids and they're two consenting adults - who's being hurt?