Complications arise because as we all know, there are various types of aunt/uncle and niece/nephew relationships (great-aunt and great-nephew is one example), and there are certainly many types of cousins.
What we popularly term “first cousins,” for example, are the children of two siblings, but there are second- and third-cousins to contend with as well.
The “direct line” refers to direct descendants — parents, grandparents, children and grandchildren.
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 risk of giving birth to babies with genetic defects as a result of marriages between first cousins is no greater than that run by women over 40 who become pregnant, according to two scientists who call for the taboo on first-cousin families to be lifted.