I am so sorry that you are going through this.

We went through this right before our Nov. 2011 trip. We bought
trip insurance when we booked, but were worried about coverage, as my FIL had been ill for a while. We never found out if it would cover or not, as he wound up passing away after just 3 days in Hospice, about a month before our trip. He told us to go ahead and go, so we did. In our case, it was healing for the kids.
As other posters have said, don't worry about the Disney Deposit, just pick any random date in the future to move it to, so you won't lose your deposit. As for Air Tran, you will have a year to use your credit. I was also advised that if we had to cancel to let them know the situation..apparently in some cases, if the death happens close to the time of the trip they will waive the change fee. Another option is to look for "cancel for any reason" trip insurance. You can find these at insuremytrip. Just weigh costs carefully....the policy may well end up being more than the $500-$600 you would pay in fees to the airline.