In the novel Elephants Can Remember, Gracie was the sister of Mrs Matcham. Gracie had cancer and passed away the previous autumn. They had operated on her, but it was too late.
When Ariadne Oliver goes to visit Mrs Matcham, Mrs Matcham says that she is all alone now, as her relations have all passed away, and she mentions Gracie specifically. Mrs Matcham asks Mrs Oliver if she remembers Gracie, which suggests that Mrs Oliver was also acquainted with her.