Call WDW. Tell them you cannot make the trip. If you don't want to pay a cancel fee, and they won't waive the cancel fee for you based on the circumstances, try changing the dates to sometime in the future. I don't know if you're DVC or not but if you are call Member Services. Your points will go into holding but you won't lose them.
Anything you have booked, like tours or restaurant reservations, that required a credit card guarantee or deposit, or prepayment, make sure you cancel those right away so you can have the prepayment refunded or avoid no-show charges.
There isn't much you can do about the airline, if you are flying. If your airline charges a cancel fee it's likely you will be required to pay it. Ask about bereavement policies, but I'm not sure that will result in a full refund, depends on the airline. If you have trip insurance, see if the insurance will cover a full refund.