I simply cannot find the answer to this anywhere. My understanding is that my father in law and mother in law are my parents under the Catholic Church and therefore the 4th commandment applies to my duties and obligations towards them just as it does towards my natural father and mother. There is a lot of information about a relationship between a brother in law and sister in law being incest, but I can’t find anything where Catholic teaching or Catholic saints write specifically about positive teachings in regards to a Catholic’s obligation towards his in-laws. Or am I wrong in assuming the equivalence between a parent and parent-in-law? If you could kindly point me towards a book or website that may have information about this that I could look up myself, that would be more than enough, but I would be grateful for any information about this topic.
ANSWER FROM Fr. Tim Ferguson (a frequent commentator here):
The Catechism of the Catholic Church treats of this matter, somewhat, in paragraph 2199. This paragraph says,
“The fourth commandment is addressed expressly to children in their relationship to their father and mother, because this relationship is the most universal. It likewise concerns the ties of kinship between members of the extended family. It requires honor, affection, and gratitude toward elders and ancestors. Finally, it extends to the duties of pupils to teachers, employees to employers, subordinates to leaders, citizens to their country, and to those who administer or govern it. This commandment includes and presupposes the duties of parents, instructors, teachers, leaders, magistrates, those who govern, all who exercise authority over others or over a community of persons.”
While it doesn’t specifically mention in-laws, I think one’s obligation of respect toward them is spelled out in this paragraph.
Further, the Church places before us in Sacred Scripture the Book of Ruth, which tells the story of Ruth, who, after the death of her husband, follows her mother-in-law in the spirit of filial piety and for her fidelity is rewarded with a new husband, with whom she bears the grandfather of King David. The Church Fathers (particularly St. Ambrose, St. Jerome, and Origen) write about Ruth as a model of the Church and as a prefigurement, in the Old Testament, of salvation being offered to the Gentiles.
Lastly, Christ’s own obedience and respect toward his foster father, Joseph, while not in the same category as a father-in-law, gives us an example to follow of showing the respect that is due to all those who are in a position of authority as well as respect to our elders, be they our parents, parents-in-law, foster parents, or other older relations.
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