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.
Agreed. And, according to the teachings of the Church, the action is “invalid marriage”. Which, as you remind us, has consequences.
Yep! That’s what makes this such a difficult situation!
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.”)
That’s a totally valid approach! And, this ‘pastoral concession’ is something that many Catholics decide to adopt!
So, Jesus would have said, “go and sin some more”?
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? )