I had a son with my first wife and two daughters with my second wife. My daughters have met my new wife, but my son (who lives overseas) has not. Nor has he met either of my wife's children, both of whom are adults. My son is getting married later this year. He has invited my wife and I to the wedding but has not invited my wife's children.
Over a decade ago, I started my business, Bridesmaid for Hire, as a way to support people during the stressful and exciting moments of their wedding. A lot of my clients have other people in their bridal party, so it's my job to help everyone else do what they're supposed to without any of the drama, headaches, or arguments that often occur.
You are not obligated to invite this couple to your wedding. Write them a sweet note explaining that you are sorry you won't be able to attend because you will be on your honeymoon at that time.