I would not be able to enjoy myself at WDW knowing what was going on. But I also have a good relationship with my DSIL, and would want to be there.
We had a WDW trip planned and had to postpone it when DMIL was diagnosed with terminal cancer. Our theory was that WDW cost a lot of money and why go if you are not able to wholeheartedly enjoy it? And truthfully, our hearts and minds would have been home, we'd have been calling multiple times per day to check on thinhgs and so forth. We postponed the trip and went after she passed away, and had the most beautiful weather and relaxing week ever. We joked that DMIL "arranged" the weather for us, since we had sacrificed our trip earlier to care for her.