The point of disagreement was that they weren’t willing to accept that not everyone believes the things they do. They hold everyone to what THEY believe. It is unkind. We define marriage differently.
Fair enough. But, the OP’s case seems to be different than yours, so you seem to be extrapolating inaccurately.
The OP’s future S-I-L identifies as Catholic. So, this marriage represents a departure from the teachings of the religion that they have in common. It’s not “they define marriage differently” – rather, it’s that they share a common faith tradition, but his brother and S-I-L are departing from the teachings of that faith tradition.
It doesn’t work that way. Actions have consequences.
Agreed. And, according to the teachings of the Church, the action is “invalid marriage”. Which, as you remind us, has consequences.
It is one thing to disagree on something, but it is a whole other matter to risk your entire relationship with a close family member due to a disagreement.
Yep! That’s what makes this such a difficult situation!
As far as I can tell, the Catholic church has never issued any guidance on it either.
Years ago, the answer would have been “don’t attend.” (I’m assuming that, since your future S-I-L isn’t free to marry in the Church, then the wedding isn’t at a Catholic Church. Maybe another venue? Another denomination? A secular celebrant? That would have led (years ago) to the answer “you shouldn’t attend.” The most recent canon law doesn’t make that assertion, so you’re right – there
isn’t a single “Catholic answer.”)
It is up to me to love them the best way I know how, despite disagreement, and that way is to attend their wedding and continue to take an active part in their lives.
That’s a totally valid approach! And, this ‘pastoral concession’ is something that many Catholics decide to adopt!
I believe that is the most “Catholic” answer and exactly what Jesus would have done.
So, Jesus would have said, “go and sin some more”?
You’re still assuming the worst about her (that she didn’t receive an annulment).
Good point, but I’m not sure it’s valid in this case. Anyway, you’re ignoring the fact that a Catholic man is, ostensibly, getting married outside the Church. (After all, if the wedding was taking place in a Catholic church, the OP wouldn’t have assumed that she didn’t have an annulment… right?

)