Your family and your DMIL are in my prayers.
Six years ago, my MIL was diagnosed with inoperable lung and bone cancer. She went through radiation and chemotherapy, and had her share of ups and downs. But, she TOLD us she was taking us to WDW the next spring, and she did it. She didn't feel all that great, but good enough to have a good time, and we all did too. She used a wheelchair everywhere, and the CM's were ALWAYS wonderful! It was our first trip, and I've taken my two sons back twice since, with another planned for next March. Cancer is hard not only on the family member battling the illness, but on the family around her helping her deal with it.
My prayers are with you all, and wishing you a wonderful and blessed trip.
Dave