In the novel N or M?, Aunt Gracie is a relation of Tuppence's. She does not appear in person, but is mentioned.
Aunt Gracie lives in Cornwall, and when Tuppence goes to Sans Souci, she tells her children that she is going to Cornwall to be with her.
Tuppence tells Miss Minton and Mrs O'Rourke about Aunt Gracie, describing her "indomitable spirit, her indifference to air raids and danger, and her vanquishment by illness". However, it is not clear how much of this is true of the real Aunt Gracie, and how much is part of Tuppence's cover story.