guess its only a problem if you want a baby together, what with the mix of genes to similar etc
It's not a problem, otherwise it wouldn't be bloody legal. My cousin took extensive advice before embarking on a relationship, if there had been any suggestion of a problem with children they would not have got married. No mad fling here, they were serious from the off.
Any 'problems' come from marrying within families going on for generations, not just the odd time. This thread just shows how ignorant some people can be about things they know nothing about. As per the norm here.