If the family is really set on having a large party like the 40 yr WA - then simply let them know that at this time you would not be able to contribute in the same fashion. You may make a modest donation or offer to take on some "service" type tasks.

No need to explain that you want to take a vacation - your business - not theirs. It is certainly ok to take a well needed vacation then spend a lot of money celebrating at a party that isn't part of your plan for the family.

Anyway I hope this gives you a starting point...