Gellchom, I know you said upthread that everyone is different, and communities do things differently but I'm surprised that you invited your cousin to your son's wedding on her own wedding day unless I've read that wrongly. People indeed do things differently!
Well, actually, more the other way around -- he had that date before she was even engaged.
I wrote about it here. http://www.weddinghellsbells.com/smf/index.php?topic=89624.0
She had known about my son's date and then months later scheduled her own wedding for the same day (which we found out when she asked my mother to help her pay for a big wedding). I assume she just forgot or got mixed up about the dates, and she did apologize. Of course no one owns a date, and though we weren't thrilled, we weren't angry; my problem for which I turned to ehell was just how to make it easier for all the other relatives who were placed in the awkward position of literally having to choose between their relatives. And it all worked out fine, I'm glad to say.
Anyway, both couples sent each other invitations, for exactly the reason I said above: "even though we know you can't come, we wish you could." (Sending each other save the date cards seemed funny to me, though!)
I think people have asked the question here on ehell whether you should send invitations to people you know can't come. Most people say yes, if you would have otherwise, I believe. But I think that's another thing to decide case by case.