These things happen to test us. I remember with husband number one..every blasted time we got shipped to a new duty base, he was sent somewhere else for 3 months to far longer. Guess who had to do the packing and all the scut work? I learned how to move freezers and fridges (you put them on upside down carpet pieces and put ropes wound through the fronts) and how to clean the base housing for the white glove treatment exam.
Now sit down and have a good cry like others said, then call your SIL and ask for help. Lots of folks are eager to help but don't want to intrude. So ASK. Tell family what has happened.
I'll bet the same or worse has happened to them. Have a "Horrible Time" contest and give a booby prize to the worst!
Now go on and have fun at your own party. Take pictures. Get some cheap "point and click" cameras. They take real good photos nowadays.