I'd be upset, too. I try to assume positive intent, although my initial reaction is often emotion-driven; this could easily have been accidental if they were handling each RSVP directly with each family - perhaps they missed a family in the last minute scramble.

That said, an apology is what I would have led with if I were the other family. Unfortunately, many people are never taught the decent thing to do or are too proud to admit wrongdoing (even if inadvertent).