Question:
Answer:
In general, a Catholic could officiate at wedding of two non-Catholics, provided there are no obvious impediments, for example, a prior marriage.
In addition, although your niece and her fiancé are to be commended for desiring to make a lifelong commitment, they will marry invalidly, because they are positively excluding an essential element of marriage: openness to children. And because it’s an essential element, this applies to even a natural marriage—that is, the type of marriage your niece and her fiancé will be attempting—not just simply to sacramental marriage between two Christians.
As the Catechism of the Catholic Church provides:
Unity, indissolubility, and openness to fertility are essential to marriage. Polygamy is incompatible with the unity of marriage; divorce separates what God has joined together; the refusal of fertility turns married life away from its “supreme gift,” the child (GS 50 § 1). (CCC 1664, emphasis added).
The Church’s Code of Canon Law affirms:
The internal consent of the mind is presumed to conform to the words and signs used in celebrating the marriage.
If, however, either or both of the parties should by a positive act of will exclude marriage itself or any essential element [i.e., good] of marriage or any essential property, such party contracts invalidly (1101, §§ 1–2, emphasis added).
Again, the Church means categorically excluding children, as opposed to perhaps delaying having a child, and that point might help persuade your niece and her fiancé.
Consequently, I encourage you to lovingly tell your niece and her fiancé that they need to be open to children to marry validly, and thus to have you officiate as a faithful Catholic. I realize that’s a “big ask,” given how they’ve been raised and otherwise formed by the culture. But speaking the truth in joyful charity is key and distinguishing between categorical exclusion vs. limited delay re: openness to children could win her and fiancé over.
If your niece still refuses, then let her know that, out of genuine love for her and fiancé, both for their temporal well-being and concern for their everlasting good, that you will charitably decline officiating.
I’d still try to be a part of her life, which she may not open to, at least for a while, but you’re the kind of person who could plant good seeds in her life over time. God bless your efforts abundantly! (John 10:10; Rom. 8:28).