Imagine being 34 and knowing that you've probably lived the majority of your life. Imagine knowing you have a genetic mutation for familial early on-set Alzheimer's and that you'll have 100% chance of getting the disease and a zero chance of surviving it.
The McIntyre sisters are facing this. One has the mutation and the other does not. They have already lost their mother to the disease and know what they're facing. They've already started planning for it and know they will have to deal with one outliving the other.
In reading the above linked article, I have to commend them for their realistic approach to the disease and what they're facing. One sister has committed to caring for the other.