Is a catholic marrying a non-catholic a sin?

My daughter is not catholic and is marrying a catholic man. His brother asked to meet with them and told them that God will not honor their marriage, it is a sin for them to marry and the bible supports this. I think his brother’s concern is misguided and the fact that he would put his belief on them in the form of guilt and shame is not right.
Where is there scriptural support for this idea?

I have no idea where this is coming from. Either you have the story wrong or the brother is very wrong.
A Catholic is free to marry a non-Catholic, the Church only asks that the Catholic party affirm that they will remain Catholic will raise any child of the marriage as a Catholic.

There is a process that is to be followed when a Catholic marries a baptized non-Catholic or an unbaptized person. As long as that process is followed then the marriage is indeed valid. If the Catholic party does not follow the process then the marriage would be invalid.

As long as the Catholic party is following the rules/process of the Catholic Church then there is no sin and everything is valid.

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