Catholics can't marry their sister-in-law?

My Greek Orthodox teacher claimed that Catholics cannot marry their sister in law. I am not sure how true this is. This could have created complications in my situation. I was dating this girl and my sister is dating her brother. If my sister married her brother my girlfriend would have became my sister in law. Would it be sin if I married her in this hypothetical situation?

Well, no, if only because she wouldn’t be your sister-in-law; your sister’s new husband would be your brother-in-law, which is not quite the same thing. As for the not marrying your sister-in-law business, that’s a new one on me. I know it used to be the case a very long time ago, but I’m pretty sure it’s gone by the board now.

I think it only applies to a situation where, for example, you had a brother and he died, and you decided to marry his surviving wife. I believe that would not be allowed. But I’m pretty sure you can marry the sibling of your brother-in-law.

Someone correct me if I’m wrong.

This is not correct; a member of my family had a husband who died, and later, her deceased husband’s brother had a wife who also passed away. This member of my family married her deceased husband’s brother in a Catholic ceremony over 30 years ago. Unless things have changed, I believe that there is no such rule that does not permit marrying one’s brother-in-law/sister-in-law.

She could have only done so with a dispensation from the bishop. Affinity is still an impediment to the second degree.

This is called an impediment of affinity. In older codes of canon law, the impediment of affinity was much more broad and restrictive than it is today, but it could be dispensed.

In the GO, they also have an impediment of affinity but also dispense it, BTW.

Today in the CC the only impediment of affinity is in the direct line.

Can. 109 §1. Affinity arises from a valid marriage, even if not consummated, and exists between a man and the blood relatives of the woman and between the woman and the blood relatives of the man.

§2. It is so computed that those who are blood relatives of the man are related in the same line and degree by affinity to the woman, and vice versa.

Can.* 1092 Affinity in the direct line in any degree invalidates a marriage.

Thre is no affinity in the direct line (parent/child/mother-in-law, son-in-law, etc) therefore, no impediment.

Your relationship of affinity, should either of you marry your current girl/boyfriend, would be in the collateral line.

Well, in the hypothetical situation in which affinity in the collateral line (such as pre 1983 code of canon law) invalidates an attempt at marriage, yes it would be invalid and therefore a sin to enter such a marriage without a dispensation from the bishop.

Well, maybe.

The new code came out in 1983, which was 31 years ago. So if they married in 1983 or after, then no dispensation was needed. The poster is not clear on exactly what they mean by “over 30” years ago.

If it was before 1983, then yes, the 1917 code included affinity in the collateral line up to the 3rd generation so a dispensation would have been needed.

Not true. See here:

Affinity would apply to the surviving wife AND her siblings. Affinity is created between all members of the spouse’s family and the spouse.

But again, it is only an impediment in the direct line-- father-in-law marrying daughter-in-law, etc.

Honestly, I am not sure what point your GO friend is trying to make, because the Orthodox have a much more restrictive impediment to marriage based on affinity:

I am sure, however, it is regularly dispensed.

So, if in the future all of us wanted to get married then we would have to go to the bishop because than this would be sin. Correct? I am not clear on why this is a sin if there is not direct blood linage. Why does the Church have this law?

What happens when spouses die–does the affinity still exist?

No, not correct.

Under the current code of canon law, affinity in the collateral line is not an impediment. You need do nothing.

It would be a sin if the Church still had such a law and you broke the law by not obeying the Church. You asked about the hypothetical situation in which there was an impediment. In such a case, a dispensation would be needed.

But, your situation requires no dispensation.

The Church had, and still has, this law in part based on Scripture which prohibits relationships of affinity and to promote harmony and stable marriage. When you marry, your families become related and tied together.

The Church is less restrictive now than it was in the past, when family bonds and ties had much higher levels of political implications as well.


Wow, so much out there I have no idea about. :rolleyes:

That is what is great about this site. :slight_smile:

Thank you :slight_smile: I always wondered about that!

As canon 109 says, affinity arises between the married person and his/her spouse’s blood relations. It does not arise between a man’s blood relations and his wife’s blood relations.

A hypothetical: I am related by affinity to my wife’s sister and her parents. If my wife were to die, I could marry her sister. I could not marry her mother without a dispensation (which was–and I hope, always is–pretty hard to obtain). In the past, I would not have been able to marry her sister without a dispensation (which was not hard to obtain).

My brother, however, is not related by affinity to my wife’s sister. They can, and always could, marry. My brother can, and always could, marry my wife’s mother. My marital choice does not legally limit my brother’s.

So, to the OP: your scenario is not about affinity.


At one time, affinity did beget affinity.

I thought that it did in the 1917 code, but obviously not if you are saying it did not.


You’re right–I should not have said “always.” That was careless of me. I was only intending to speak about 1917 and after, and wasn’t even thinking about pre-1917 law. In that regard, there were some times and places where affinity begot affinity but that was done away with in the Catholic Church by the Fourth Lateran Council (1215). At least, that is what I have been told.

As far as the 1917 Code, it defined affinity in the same way as the 1983 Code: it arises only between the husband and the blood relatives of the wife, and vice versa (1917 Code, canon 97). That Code did extend the impediment to what it called the second degree of the collateral line (which, I think, would have meant first cousins) and also did speak about affinity “multiplying” (c. 1077). Frankly, I don’t understand that very well but it doesn’t change the meaning of what affinity is in that Code.


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