Are they getting married in the Catholic Church? If so, then definitely yes.
they did not know the priest there, and I don’t know if he is aware of their current living arrangements.
Maybe he is. Maybe he is not. That’s not your business to be quite frank. Don’t assume that he doesn’t. My best friend and his wife lived together before marriage. I was not a fan obviously but I didn’t breathe a word about it. I just assumed that the priest who was marrying them probably knew about it. And guess what? I was right! And as it turns out, my buddy admitted to me that he regretted living with her and that they actually went all out to live as “brother and sister” during the wedding preparation time.
This was not information I desired to know to be honest. I definitely didn’t ask. He volunteered it. It didn’t make me feel better or worse about attending the wedding. They were getting married in the Church and that’s what mattered to me. If they were
not getting married in the Church, maybe I’d feel differently.
My brother-in-law is getting married pretty soon. He’s not living with his fiancee but they have taken many trips together. Should I assume they’ve done something wrong and that I should lodge some kind of protest? No.
I traveled about 1,000 miles to visit my now-wife in college and she did the same for me. We roomed together during those visits because there was nowhere else for us to go. Guess what? Nothing happened except for some kissing. Seriously. So why should I assume anyone else would do anything different?
I’m not an idiot and I know that there’s plenty of opportunity for others and I know full well that tons of people live together before marriage complete with sex and are 100% OK with it. But that doesn’t give me the right to assume that in every or, actually,
any instance until it’s proved otherwise. Maybe your niece and her fiance have spoken with priests who have advised to live chastely and they’re doing it. Or maybe they are at least trying and have confessed their sins. Maybe not. You don’t know.
So unless they are getting married in a church but not
in the Church… imo you should probably go provided that it’s not a major inconvenience. Once they’re married in the Church and confessed their sins, their previous potentially sinful situation is
over.
Incidentally, my buddy’s wife is now an NFP instructor and they’re probably the best Catholic couple my age that I know!