I mailed out our invites on July 10th and what do I pick up from our mailbox on Friday? One of our invites has been returned, with a week until the response date and 3 weeks til our wedding! The invite was sent to Texas (from Virginia) but I'm shocked that it took five weeks to get back to us. And now we're not really sure what to do. 
It's at least partially our fault as well; we saw these folks (FI's second cousin, his wife and daughter) at another family wedding in April and they mentioned that they would be moving this summer. When we saw FI's great aunt in late June we asked if they had moved yet and she said no but we should have followed up with them before we sent out invites (2 weeks later). I immediately sent a Facebook message to the wife when I checked the mail on Friday (we don't have their phone numbers or emails). I apologized for the mix up and asked for their new address so we could re-send the invite. She saw the message but hasn't responded. *Sigh* 
FI texted his grandma this morning to get one of their phone numbers but I feel bad about this. At this point there's probably no way they can attend our wedding, what with putting in vacation time at work, booking flights and a hotel. And weddings in his family are basically family reunions so I feel like they would have attended if they got the invitation in a timely manner. Is there anything else we should do/say? Second cousin's wife has a bit of a reputation for being touchy and I'd hate for them to think that we uninvited them.