Since people want a "Mormon" response, I'll weigh in.
You have a Mormon relative. You intend to have your wedding in your congregation's building.
Do you invite your relative?
If your answer is "no", then how is that any different?
I don't know anyone who wouldn't invite a non-member to a Christian wedding. Every wedding in my family in our church was attended by Christians of other denominations as well as non-Christians such as Jews, Muslims, atheists, and yes, even Mormons.
That's the whole point of this thread - the practice of exclusivity is specifically mormon and the irony is that the mormon religion, above all others, likes to tout that they are very pro-family.
Years ago my sister worked for A&P (I can tell this story now because they're out of business). They were a huge grocery chain and my sister worked in the corporate headquarters. At one point, they were experiencing severe problems with quality and freshness in their produce, chain-wide. What was their response? They launched a new advertising campaign that boasted that fresh produce was their #1 priority. They boasted the exact opposite of the truth about their business.
That's how I see the mormon religion. I always believed they were very pro-family but now I realize it's because they SAID so. When you have to say it, it's usually not true. Now I see that. They SAY it so much but it's a cover-up.
So to answer your question, YES I invite the mormon relative. Or even a mormon friend, or acquaintance, if I feel so inclined. My Church would have no problems, they don't ask, they don't want to know anything about the religions of people attending. Our ceremonies are all to the Glory of God and the Church sees it as another form of evangelizing. Our Church and our priest would LOVE to see non-Christians come to the weddings as it gives us a chance to share Christianity with them. We have no hidden ceremonies or rituals or anything. Everything we do is in the open.