Edited on Thu Feb-14-08 06:42 PM by Mr. McD
Double cousins
Generally, one's cousinship to another is determined by a connection through only one parent's biological family. But an individual's cousinship to another individual may be determined by a connection through both of one's parents. These cousins are biologically connected to both the maternal and paternal family trees and that cousinship is termed a double cousin. Another term used to describe this is cousins on both sides.
If a pair of siblings from one family each form a couple with a pair of siblings from another family, then the children of these two couples will be double first cousins to one another. The children of the couples would already automatically be first cousins due to the fact that they are children of one of their parent's siblings, but in this case the children of their mother's sibling, are also the children of their father's sibling, and thus they are double first cousins. Such cousins have double the consanguinity of ordinary cousins and are as related as half-siblings. Instead of the 12.5% consanguinity that simple first cousins share with each other, double first cousins share a 25% consanguinity with each other. Further, if identical twins form a coupling with a corresponding set of identical twins, the children of these two couples, though legally (double) first cousins to one another, would genetically be as closely related to each other as ordinary full siblings.
Sometimes the children of these unions are called cousin-siblings, cousin-brothers, or cousin-sisters. Note that no inbreeding has to occurred to create these close kinships.
http://en.wikipedia.org/wiki/Cousin#Double_cousins