some people with hereditary illnesses have children knowing full well there is a high chance they will grow up with the same ailment. one of the queens of england married a russian monarch knowing full well there was a history of haemophilia within the bloodline. Can't remember for sure but I think one of their kids died early as a result.
IMHO incest is just plain disgusting. no need to rationalise why it is wrong, some things we just know.
