From a reader…
My wife’s brother is getting married in Salt Lake City this January in a civil ceremony. He is a non-practicing Mormon and his fiancee is also (as far as I know) a non-practicing Mormon. The fiancee was, however, previously married. My wife and I would like to be able to say yes to the invitation, but we have had qualms of conscience about whether we should attend a wedding in which one of the parties is a divorcee. I realize that neither of them is a baptized Christian, but nevertheless, my understanding of the teaching of the Church (from Pius XI’s Casti connubii) is that even so-called natural law marriages are indissoluble, by divine institution. So, we’re puzzled about what we can do in good conscience — especially since we would have our children (15 and 12) with us.
A couple points.
Mormons are not Christians. They do not have valid baptism. They do not understand the word or concept “Trinity” in a Christian sense. Frankly, we are not overly concerned about how Mormons marry… apart from the whole polygamy thing.
That said, from what you wrote the planned civil marriage would probably be invalid because of the prior natural, not sacramental, bond. However, there is nothing in Canon Law that prohibits you from attending an invalid wedding. No sacramental issue is at stake here. This is not a situation of a clear violation by Catholics of Christian, Catholic doctrine or discipline that might cause scandal. Family presence might keep the door open for future conversion. You can go.
Explain clearly to your children that your presence is more about showing support for a family member.