OK because no child should arise but even if accepted a brother and a sister then have a child what would be so bad about it? I do not know why siblings should fall in love with each other but even if that works, I would not think that bad now.
This has to do with genetics and genuine trouble and is quickly complicated. But here's a beginning :) Inbreeding in humans - Wikipedia
Because then gene mistakes become more likely, that is, the likelihood of disability increases (dramatically) ,
Because by the closeness, the risk rises to get children who have a disability or other health problems.
And go wrong,
The danger that the child comes sick to the world is seemingly big, whereby incest is actually banned only in Europe. But what prohibited is well known to be particularly appealing.
Because we are thingCriminalizing "obviously" incorrectly appeared (culture dictates right).
In the case of incest, it is assumed that the intersection is at risk by nearly relatives the health of children. In a completely rational world siblings would be genetically tested. If the results justified this, they would agree not to give birth to children.
Note that we do not impose any people in which the risk of birth defects exists by smoking or drinking.