Respondez s'il vous plait translated in English is "Reply, if you please." I've always understood it to be customary to reply to an RSVP request whether you are confirming or declining an invite. It simply means that the invited guest must tell the host whether or not they plan to attend the party. It doesn't mean to respond only if you're coming, and it doesn't mean to respond only if you're not coming - it would say "regrets only" for that. It means the host needs a definite head count for the planned event, and it needs to be by the date specified on the invitation!
So, what do I do with these 60+ family members who have not done this simple task a week after the specified date?? Planning a family reunion that involves 96 people is no small task, and this is the part that I thought would be the easiest! What else is in store?