To me, one of the biggest and most important functions of portraits is to keep people, as it were. A photograph may very well outlive the physical person, and as such it has a personal significance to it not found in other types of photography. These images may never bring happiness in and of themselves, but the fact that they exist will serve as a reminder of something the family apparently never wants to forget, and that is good. It will be a hard thing to do, but I know you will do a wonderful job.