first and foremost, if you truly can't take the time off, then you can't. period. end of discussion.
if your MIL has a package deal onsite at disney, it is too late to get a refund for any member. the entire package would have to be cancelled and a new one made and paid for. but you would not get a refund. and you would lose any discounts you had.
if your MIL has a room only onsite at disney, you have 72 hours to cancel, if you were a third adult in the room, you only cost about $25 per day, but again, it may require cancelling and rebooking -THIS IS A QUESTION FOR MASTER YODA. still working on the room only idea, if your ticket has been purchased, someone can use it some other time. just remove it from the website if you registered it and someone else can claim it someday- but you are burning that money until someone else uses it. dining reservations that you do not go to may choose to charge the $10 no show fee for you, and for prepaid meals, most have a 72 hour cancellation or you lose the money.
HOWEVER
I, as a fear of flying person, suspect your fear is what is really pushing you over the edge on an otherwise not so dire situation. I always get panicy a few days before i actually go.
i did not fly for over 20 years because of this fear. my family would fly down and i would take the train and meet them. until I accepted the fact that if i really believed that plane was going to crash, i would not be putting my daughter on it. so deep down, i believed it was safe. i was just afraid to be on that plane!
so i decided to try the heavily drugged route. alcohol was not enough and it gave me a headache. so now i take a relatively large dose, and I mean large dose, of xanax and get on that plane and go. i take enough that i once told my daughter i thought it was strange that they don't do the safety talks before the flights anymore. she pointed out that i am unconscious by that point, they still do them.
not a bad way to fly.
see your doctor, get a presciption and go with a family that wants to share their joy with you.