First of all, do you know for a fact that your nephew did not have a dispensation? If it’s possible that he could have a dispensation, the charitable assumption would be that he did and thus that the wedding was presumably valid.
If you know for a fact that he did not have a dispensation and that the wedding was presumably invalid:
Yeah, I know for a fact. The problem is, he doesn’t practice his faith now, nor did he then…he lived with his now wife, back then…so my sister said it wasn’t something that ínterested him…(getting married in the Church) They are going through an awful divorce now (is divorce ever not awful?) so if he married in the Catholic Church, would he need to have a decree of nullity for the first marriage?
The argument about whether or not it’s a sin to attend depends on your assumption of whether or not an attendee implicitly supports the marriage by being at the wedding. Personally, I believe that if you attend a wedding, even if you’re just sitting in the back of the room, you’re approving of the wedding. Therefore, attending a wedding that is presumably invalid would be scandal. However, there are others that believe that just being in the same room as the wedding is not an implicit approval of the wedding. Thus, being in the room is not scandal. That is where the disagreement (and confusion) seems to lie.
*I tend to agree with you now, but back then, I was cloudy on the issue, I hadn’t thought of my own culpability in attending my nephew’s wedding.

I only thought of him marrying outside of the Church. On many levels, what you say makes perfect sense…because when I attend weddings, I’m sort of celebrating that occasion with the bride and groom, so true. *
Others seem to confuse the argument by trying to make an emotional appeal: if you don’t attend, it might cause severe hardship to the couple. I really don’t buy this argument. If you truly believe what she’s doing is wrong, and you truly believe that her soul is in danger because of it, how could you support it?