Catholic church and dating
The list of collateral relationships goes on and on!
To make the picture even more confusing, the Code of Canon Law is based on old Roman law when it comes to family relationships, so it doesn’t use our common terminology.
In accord with the same canon 108.3, we count all persons involved in this relationship, except for the common ancestor: Susan, Bill, and Mary add up to three.
Knowledge of the meanings of these two terms should help us now to understand canon 1091.1: marriage is always invalid between parent and child, grandparent and grandchild, great-grandparent and great-grandchild, etc.
So these two cousins — who in our parlance are “first cousins” — are related in the fourth degree of the collateral line. Not according to canon 1091.2, which says marriages are invalid up to the fourth degree.
First cousins, therefore, cannot marry in the Church.
Canon law is therefore simply reflecting what nature (i.e., God) intended.
A casual reader of canon 1091, the canon which directly addresses Jeremy’s question, will most likely find it hopelessly confusing.
To begin with, it would be helpful to clarify some terms.