In chapter 1 of "The Chrysalids," who is David's sister?

In "The Chrysalids," David Wyndham opens the novel with narration from David who is describing dreams that he often has.  He tells us that he shares these dreams with his oldest sister, Mary.  She tells him that he there are no such places but he could be dreaming about things or places that existed long, long ago.  She also warns him not to tell anyone else about these dreams because she doesn't know of anyone else who has dreams like that and he would get into trouble if anyone thought he was "different." (pg 5)

Later in chapter one David also tells us about his other sister Sarah who is 15 years old.  He tells the reader that because there were so many years between them that he often played by himself. (pg 6)