Last Minute Cancelations...the week of the Wedding

Just be blunt. At this point it is week of and you will have bigger fish to fry. Have your FH tell his cousins that you were told through the grape vine that they aren't planning to be there for the dinner so the table is being removed from the set up and to have fun at the graduation. I'm sorry...My wedding is saturday and I have one guest that has now said they are unable to come. I can't imagine 12. That is rude...but let FH handle its since it's his family.

That's incredibly rude of them, especially if they already RSVP'd yes. Can't they go out to dinner the next night to celebrate the graduation? Tell them you need a firm answer now in order to make proper arrangements, and that if they are coming after dinner, they will be no designated seats for them. Don't worry about sounding like a jerk.

I would confirm with each guest their expected arrival time. Have a discussion with each of them, so the plan is clear. If they say they will arrive after dinner, and you're OK with it, then invite them to drop by for a drink. I would make it clear that if they then change their minds and expect to show up for dinner, they won't be accommodated.

They can't expect you to have a table and dinner paid for in case they decide to show up later, and I would politely say no to that.

Have your fiance tell his cousin, twelve (12) last minute threatening cancellations are costing you both money. It's time this cousin gets in the real world. This cousin doesn't expect a graduation gift from you, does he?

Oh wait, what was the final deadline with your caterer? Sometimes it's a week before without money lost.

That's super rude and inconsiderate, and I'm really sorry you guys are having to deal with this right now. I don't have any good advice to offer, but I will say that sometimes (especially those who haven't been involved with planning a wedding in the last decade) are completely out of touch with the per head costs associated with a wedding/catering headcount deadlines and they honestly might be clueless about how much money they're costing you by this last minute decline. 12 people really is a substantial amount of people, and that's a LOT of money

Just taking away their table? I'm a big enough "jerk" to ask them to pay for their meals. It's not like this is some random friend or distant family member, these are immediate family members. They already had a prior engagement (your wedding) when they decided to change their minds and attend a graduation, instead. I'd tell them that since they are attending the graduation instead, you'd appreciate they pay for the very expensive food that they will not be eating anymore. Asking that might inspire them reevaluate their choices.

Sorry this is happening, and best of luck to you both!

I would not feel bad about taking away the table, that was rude of them to decide this last minute. Also, if they already had dinner, then they can just go dance or ask to share a seat with someone who is out dancing. Not your problem anymore! You have enough on your plate.