Miss Manners: They invited you to their wedding … they didn’t ask you to walk the plank

DEAR MISS MANNERS: My wife and I are close friends with another couple. We have traveled locally, domestically and internationally with them. We have been on countless outings, and are closer to them than we are to our own families, save our parents.
They are among our truest, dearest friends. Our relationship with them means a great deal to us both, and we cannot imagine not having them as part of our social lives.
We have been informed that the recently engaged sibling of one of them intends to invite us to their wedding. I find this odd. Over the past decade-plus, the number of occasions that we have been around this sibling can be counted on one hand. We have never received any invitations from this person in the past, we have had very few direct interactions with them, and we’ve never even met the spouse-to-be.
Given the increasing prevalence of gift-grabbing in the guise of group gatherings, I cannot help but think that that is the motivation for including us. I have no interest in attending this wedding, nor in providing a gift.
What is your recommendation for declining an invitation from the sibling of a very dear (and very sensitive) friend? Or should I bite the bullet for the sake of this long-term friendship and spend a day with gritted teeth? Am I obligated to give a gift either way?
GENTLE READER: My goodness. Miss Manners would have thought they told you to walk the plank rather than invite you to their wedding.
If you hate the idea of attending that much, politely decline. But would it not be better for everyone if you assume that the sibling of your dearest friends invited you because they consider you family — and not as a gift grab?
(Please send your questions to Miss Manners at her website, www.missmanners.com; to her email, [email protected]; or through postal mail to Miss Manners, Andrews McMeel Syndication, 1130 Walnut St., Kansas City, MO 64106.)