Mary-Love made a very important comment.
The only thing they are going to see is some rigid, unbending, religious fanatic throwing a wet blanket on their day. Good way way to win converts and reverts - NOT.
I hope that people read and re-read what she wrote because that's the point. If someone chooses not to go to such a wedding - and they have EVERY right not to go, they need to be careful how say it. If in the process of being "right" we end up driving people further away then we have failed in our duty to spread the Gospel.
Of course I am not saying (and neither is she) that we are to "tolerate" every idea out there. As I said to Bob earlier, the focus should be on both truth and love. It's not one or the other.
Mary-Love also said:
And what happens when the family member marrying out of the church or without a decree of nullity hasn't got a clue as to what the Church really teaches about the Sacrament of Matrimony?
In a case like that I couldn't attend because in the eyes of the Church the person is still married. While they don't know the Church's teachings, I do. I would simply tell the couple that I was unable to attend. If pressed I would explain it as lovingly as possible. If at that point they view me, and the Church, as rigid or unbending, there is really nothing that I can do about that. However if I had called up relatives and trashed so and so, then it would have been my fault if my actions drove them further away.