>> Why not allow brothers and sisters to marry? >Claimed societal interest in preventing harm to offspring born from inbreeding. They can produce offspring without a marriage. Or, if it would be allowed, they could marry and dont produce offspring. Dont see how one is connected to the other. Its not a question of reasoning. All you need is a lobby big enough and enough time and you are allowed to do whatever you like to do. Marry a 12 year old, marry a pet, marry a car..