Collateral consanguinity is the relationship between persons who have a common ancestor but do not ascend or descend from one another. For example, the relationship between cousins who have the same grandparent, or the relationship shared between uncle and nephew.
Consanguinity is of two types: lineal and collateral. Lineal consanguinity is the relation which exists among persons, where one is descended from the other person. For example, the relationship shared between the son and the father or the grandfather. The degrees of consanguinity are taken into account in descent and distribution. In the U.S., all states consider marriage between lineal descendants illegal. And, most states prohibit marriages between uncle and niece, nephew and aunt, and between first cousins.