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Same question as here but for Catholicism. I don't see how a married couple's becoming step-siblings is any different from step-siblings' become a married couple.

Suppose Alice and Bob are widows (or single adoptive parents or unmarried biological parents or whatever) and their respective children are Charlie and Dalia. Suppose all 4 are not married (or were never married or whatever, depends if widow or not) and none of them have ever divorced or annulled.

If Alice and Bob marry, then I think Charlie and Dalia cannot marry. But if Charlie and Dalia marry, then can Alice and Bob marry afterwards?

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Q: Can co-parents-in-law marry?

A: Alice and Bob would be able to enter a valid marriage.

This commentary of Canon Law gives some background.

[Marriage] is invalid between persons related by consanguinity in all degrees of the direct line (c. 1091.1), and that in the collateral line, marriage is invalid up to and including the fourth degree (c. 1091.2).

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)."

"The “direct line” refers to direct descendants—parents, grandparents, children and grandchildren."

"The term “collateral line” refers to all relationships that at least partly involve siblings. Brothers and sisters are the clearest example of relatives in the collateral line, but collateral-line consanguinity refers also to relations between aunt/uncle and niece/nephew—as well as to cousins."

Now, in this question there is no mention whatsoever of consanguinity. The question here is of affinity. The same website has this article addressing your exact question.

A relationship of affinity arises from a valid marriage, and it pertains to the wife and the blood relatives of her husband, or the husband and the blood relatives of his wife.

Affinity is calculated just like consanguinity is, in lines and degrees (cf. c. 109.2). The relationship between a mother-in-law and her son-in-law, or a daughter-in-law and her father-in-law, is said to be in the direct line; while brothers- and sisters-in-law (as well as nieces/nephews, cousins, and even more distant relatives) are related in the collateral line.

Note that the code is completely silent about affinity in the collateral line—which means that there is no prohibition at all. It is thus possible to marry a brother- or sister-in-law, or the cousin of one’s deceased spouse.

But your question is not of someone marrying their inlaw. But of someone marrying the in-law of a son or daughter.

Affinity "pertains to the wife and the blood relatives of her husband, or the husband and the blood relatives of his wife."

Meaning that Alice and Bob have no affinity! If Charlie and Dalia were married, the only affinity change would be Charlie with Dalia's family, and Dalia with Charlie's family. The wikipedia article on affinity mentions this principle: "affinity does not beget affinity".

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    Re: 4 I have links and definitions for each of those terms... I'm not sure what else you are asking for; more detail and links are also in the articles I linked. "Co-parents-in-law" is not a defined term in Canon Law. Canon Law is descended from old Roman law, as such it defines familial relations in terms of direct/collateral line, and in terms of degrees of consanguinity/affinity. Jul 19 at 17:56
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    On 3 I'm not really sure what all you're asking. C-D marriage does not affect A-B marriage at all, whether it happens before, after, or not at all. The marriage of step-siblings is a separate question that I'll look for, but should be answered well enough by the website I linked Jul 19 at 17:58
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    1. Like I said earlier, "co-parent-in-law" is just not a relationship that exists within canon law. A man shares no affinity whatsoever with the blood relations of the spouse of his own blood relation. This is what "Affinity does not beget affinity" means. Jul 19 at 18:00
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    Step-siblings don't share any degree of consanguinity (no blood relation) or affinity (only with the spouse of their bio parent) and so it seems to me that step-siblings could contract a valid marriage. Jul 19 at 18:05
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    "Alice and Bob have no affinity", assuming they're not married to each other.
    – Geremia
    Aug 11 at 0:29

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