Guys, I need some input here. Is it ever okay to attend a wedding without a formal invitation?

Some background: So one of Mr CunnyBird's cousins is getting married. This particular cousin is really awesome, and his future wife is one of the nicest people you could ever meet. I love the two of them, even though I've only only met them once for 5 days. Mr CunnyBird used to be really close to his cousin; they were playmates as kids and used to live together in their early 20s. Lots of shared adventures. They now live on opposite ends of the country and have lost touch, but they're still on good terms and never had any falling out (as far as I know).

We knew they were planning to get married for awhile. But we found out about the actual wedding through the family grapevine. Specifically, Mr CunnyBird's parents forwarded him their email invitation. We never received an invitation of our own.

So now Mr CunnyBird is all set to buy tickets to fly across the country for this wedding. I'm totally willing to spend the money, but not if we are going to show up and be unwelcome! The wedding is a casual affair with a potluck reception in someone's backyard. The couple's personality is mildly stoner and very laid-back. They never mailed paper invitations to anyone. Mr CunnyBird and his parents are convinced it was a mistake and we will be totally welcome, but I hesitate to fly across the country for a wedding to which we were not directly invited. The obvious solution would be to contact the cousin directly and ask, but Mr CunnyBird thinks that isn't necessary and would be weird.

What do I do, GT? Am I being paranoid? Are we about to become wedding crashers?

ETA: Holy shit, guys. He just bought tickets. This is happening.