Obviously, at the creation of man, Adam and Eve’s sons and daughters married siblings. God approved this at that time for the purpose of populating the earth. People in that age were of a more stable genetic construction, and breeding between siblings did not adversely affect the children of such a union.
Even until the time of Moses, marriage between a man and his half-sister or aunt was allowed (Gen. 20:12; Ex. 6:20). Upon giving the civil laws to Moses and the Israelites, God forbade marriage between those who were closely related (Lev. 18; 20). This was to avoid the degeneration of offspring.
In modern times, marriage between first cousins is strictly illegal in most of the United States. Marriage and breeding between close relatives results in mentally or physically disabled offspring, and should not be practiced!