Roman Catechism 16.54

Chapter 16 — Part II: The Sacraments — Baptism

Section 54. Affinity Contracted By Sponsors — The Church, therefore, in her wisdom has ordained that not only the person who baptises contracts a spiritual affinity with the person baptised, but also the sponsor with the godchild and its natural parents, so that between all these marriage cannot be lawfully contracted, and if contracted, it is null and void.