Catholic Church Law is known as Canon Law. 

In Canon108 the topic of consanguinity (blood relationship) is addressed. There are two lines-direct and collateral/oblique/parallel.
"Relationship in the collateral  lines is determined merely by the total number of generations or persons -again excepting the common ancestor or root. 

 The marriage law inhibits marriages 'in the fourth degree.'" So make a family tree up to the common ancestor.

Count the number of people between the two who want to marry, subtract one for the common ancestor and you see what degree the parties are related.

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