Catholic church and dating
To begin with, it would be helpful to clarify some terms.
“Consanguinity” refers only to biological, blood relationships (and not to relationships created through marriage, such as that between a mother-in-law and her son-in-law).
That’s why the term “first cousin,” for example, is found nowhere in the code.
In canon 108, the code provides some general definitions to explain the system it does use, which involves lines and degrees.
This should certainly be no surprise to anyone, because we are quite comfortable with the notion, founded in natural law, that such marriages are forbidden.
(Natural law is, incidentally, the rationale behind the fact that these marriages are against the civil law as well.) While direct-line consanguinity is pretty easy to calculate, figuring other blood relationships can quickly become rather complex.
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.
The term “collateral line” refers to all relationships that at least partly involve siblings.