Q. A niece of my husband has become very close to one of her second cousins. They and some family members are questioning if they are too closely related to marry one another. Are they?
A. The Catholic Church’s current laws permit a marriage between second cousins, that is, between a man and woman who have a common great-grandparent.
The canonical term for family relationships too close to permit a marriage is called consanguinity. This is subdivided into direct (grandfather-son-granddaughter) and collateral (uncle-niece-great-niece). Marriages within the direct line are always invalid. Those in the fourth degree of the collateral line are invalid (Canon 1091), but a local bishop can give a dispensation for the marriage of first cousins (Canon 1078).
Under the 1917 Code of Canon Law, the marriage of second cousins was not permitted. Under the 1983 Code of Canon Law, it is. Second cousins are related in the sixth degree of the collateral line, as the New Commentary on the Code of Canon Law explains. The new Code prohibits marriage up to and including the fourth degree.
These are the Catholic Church’s requirements. Civil law can make other provisions. I am not aware of any U.S. state that prohibits the marriage of second cousins.