The scenario

A brother and sister are separated at birth and grow up not knowing of each other. Years later they meet and unbeknownst that they are brother and sister, fall in love, marry, and have children.

The question

Later, perhaps due to a search in family medical history, they discover this fact in their past.

According to the Catholic Church, what is to become of a family upon this discovery?

An answer may include a real life example.

  • Although I do not ordinarily comment, vote, or answer questions tagged Catholicism, This question is so obviously one which can only elicit answers that are speculative I felt compelled to vote to close it.
    – BYE
    Jan 15, 2015 at 20:34
  • 1
    @Bye - as a matter of fact, this does admit of a specific answer in canon law. It is not speculative at all. Jan 15, 2015 at 20:40
  • @Bye cf. Accidental incest | Wikipedia and this Google search.
    – user13992
    Jan 15, 2015 at 20:43
  • For any other denomination this question would be too much, but since Catholicism has written about so many topics, it should be okay.
    – curiousdannii
    Jan 15, 2015 at 22:02
  • 2
    @FMS Check out the Catholic Encyclopedia article "Consanguinity (Canon Law)."
    – Geremia
    Jan 16, 2015 at 5:10

2 Answers 2


Legally in most places a marriage that would have been illegal if the facts were known is automatically anulled - in other words it is deemed never to have taken place. Here is a page about it from Californian law..

Wikipedia gives grounds for automatic annullment of a marriage according to the Catholic church - it includes 'consanguinity', i.e. close affiliation by blood. Siblings would certainly count.

The anullment of a marriage does not affect the status of the children. They are considered 'legitimate' as long as at least one parent entered into the marriage in good faith. The anullment does not take away any of the duties of the parents to care for the children.

  • 3
    Real life examples of this are extremely rare, and asking for one is not going to help understand the situation. Jan 16, 2015 at 1:48
  • 'as long as at least one parent entered into the marriage in good faith' --> ah so if both of the parents tried to deceive people into thinking they didn't know about the invalidity of their marriage (for reasons of incest or otherwise) but went through it anyway then upon discovery the children would be illegitimate?
    – BCLC
    Jul 24, 2022 at 2:42

The Code of Canon Law prevents certain people from marriage:

Canon 1091 §1. In the direct line of consanguinity [blood relationship] marriage is invalid between all ancestors and descendants, both legitimate and natural.
§2. In the collateral line marriage is invalid up to and including the fourth degree [including the relationship aunt-nephew, uncle-niece, first cousins, and siblings].

While the Church has occasionally dispensed consanguinity in the fourth degree (that is, it has occasionally made an allowance for first cousins to marry), a letter of Pope Benedict XIV exists titled Aestas Anni, which states (in paragraph XIII):

Non convenit inter Theologos an matrimonium inter fratrem et sororem, jure naturali, Divino, aut humano prohibeatur. ... neque tamen ullo probato testimonio evincitur, a quopiam Pontifice admissas fuisse preces fratris et sororis matrimonii vinculo invicem ligari petentium.

That is (my translation):

Theologians do not agree whether marriage between brother and sister is prohibited by natural, Divine, or human law. [The difference is that the Church can set aside prohibitions of human law, but not of natural or Divine law.] ... nor is it demonstrated by any tested evidence that the requests of a brother and sister to any Pope at all, seeking to be bound in the bonds of marriage to one another, have been admitted.

No Pope, in other words, has ever allowed a brother and sister to marry.

Thus, the couple's marriage would certainly be considered invalid; and it would have to be nullified (the Church would recognize publicly that there was never a valid marriage). The question is, what to do now?

The siblings may or may not feel comfortable living together; that decision is theirs to make. If they so choose, it would not be a violation of canon law for them to live together as, well, brother and sister.

Although their marriage was invalid, and therefore null, they entered into it believing it to be valid; the Church calls such a relationship a putative marriage:

Canon 1061, section 3. An invalid marriage is called putative if at least one party celebrated it in good faith, until both parties become certain of its nullity.

The children of the union are therefore considered legitimate:

Canon 1137. The children conceived or born of a valid or putative marriage are legitimate.

And certainly the parents still have the obligation to care for them:

Canon 1136. Parents have the most grave duty and the primary right to take care as best they can for the physical, social, cultural, moral, and religious education of their offspring.

Whether the brother and sister chose to chastely live together in an unmarried state, with their children, or separated and made some other arrangement to raise the children, they would be considered unmarried, with legitimate children, and with the option (should they so choose) to marry a new partner in the Church.

  • Well constructed answer and has more detail regarding children but this is confusing: The Code of Canon Law allows (under a very different situation, a couple affected by the "Pauline privilege") for two formerly married people to "cohabit peacefully without affront to the Creator"—see for example canon 1143,
    – user13992
    Jan 16, 2015 at 19:51
  • Regarding Aestas Anni, the relevant passage appears to start at paragraph XIII. Jan 16, 2015 at 20:00
  • @FMS I altered that section. Comments? Jan 16, 2015 at 20:45
  • @MattGutting Thank you. I eventually understood what you had written by giving an unrelated but relevant example. But I believe your edits may have removed the confusion, which I encountered, from others.
    – user13992
    Jan 16, 2015 at 20:51

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