The question of relationships between blood relatives has been approached in many different ways throughout the course of history. There have been times when relationships of this kind have been commonly accepted. Whilst at other times they have been prohibited for moral, religious or even scientific reasons.
Relationships between blood relatives were actively encouraged in some primitive cultures in order to promote clan unity. For example, marriage between members of the same family in ancient Egypt. The legislation in a large number of US States currently prohibits and actively pursues relationships between first cousins.