Your question combines artistic, technical and emotional factors. In his 1999 book
Sight & Insight, Burton Silverman includes a painting of his mother he did from a drawing of her in her final days. One of the nicest aspects of this book is the way his thoughtful observations are laid out side by side with the paintings. He tells how he came to draw and then paint his mother in this extreme state and what it meant to him. (Both drawing and painting appear). You might want to look this book up and see how Silverman handled the evident facts of his mother's decline both lovingly and beautifully.
One of my friends is an Episcopal priest who does a wonderful job with helping families address the fact that their loved one is going to die (as are we all) by simple acts such as foot washing, talking about and honoring a life well spent. But many people prefer to avoid an open discussion. You may do the family a valuable service by opening the topic you raised, but only you can judge if they would be receptive.