Are we invited to a wedding if we received a "save the date" postcard but not an invitation?

In July, we received a postcard asking us to "save the date" for a niece's wedding. No time or place was given on the postcard. Now, 3 days before the wedding, we have still not received a written invitation to the wedding. We've not been told what time or where. My knowledge of etiquette is that we do not attend without a written invitation. But, the bride's side of the family never pays much attention to etiquette anyway, so who knows what they're thinking. My husband has verbally mentioned it to some of his siblings, but the bride and her mother have not responded one way or the other. Sounds to me like we're being set up for the proverbial "between a rock and a hard place."