My kids were 13 and 15.
At first, we simply told them mom had a bad foot, and the doctors were looking into it. I asked the kids to help out more often around the house.
Then, we said she needed a wheelchair because her leg was sick, and I asked them to help with more chores.
Finally, after they had seen the relentless progression of a disease that was obviously not going away, but they already had plenty of experience coping with it, I told them, one at a time alone, that mom was eventually going to die, and until then, it was up to us to make her life as comfortable, pain free, and happy as possible.
We cried of course. Then we embarked on our new course.
The kids, over time, learned how to feed mom, crush and mix meds, operate the patient lift to move mom around the house, operate the breathing machines, etc.
By being involved, I think it helped them process.