During their younger years, Sandy and Dennys are the "practical" members of the Murry family, maintaining what they consider a realistic view of the world. Until their adventures in Many Waters, and to some extent even afterward, they tend not to witness the extraordinary events the rest of the family encounters, and assume there must be a rational explanation for them. As adults they have a more comprehensive view of the world, recognizing both its practical aspects and the need to fight the evils encountered there. The twins are very close to each other, with Sandy usually being the leader. It is said that they seldom disagree on any subject for long.
Sandy and Dennys are overshadowed by their Nobel Prize-winning mother and space-traveling physicist father, their mathematical genius sister Meg Murry and their extraordinary younger brother Charles Wallace Murry. However, the twins are quite smart themselves. At age eleven, Sandy quotes Robert Burns correctly, and knows what a rachis is. Both twins understand the conservation of matter and Albert Einstein's most famous equation at the same age. Despite this, Sandy and Dennys are considered the "normal" members of the family rather than misfits. They get along well with their peers, are good at sports, and as children maintain a B average in school.
As of A Wind in the Door, Sandy aspires to become a banker, on the grounds that it is practical and lucrative. Dennys apparently concurs. Their career plans are refined as they get older, with Sandy studying to be a lawyer and Dennys a medical doctor.
As a child, Dennys plays the flute in the school orchestra, and is fairly proficient with the instrument. Sandy plays the piano, and later is in the glee club at college. As a young adult he is said to have the best singing voice in the family.
Before their adventure in Many Waters, Dennys is generally less skeptical than Sandy about Meg, Charles Wallace, and Calvin O'Keefe's supernatural adventures. He is also more likely than Sandy to pay attention to the family's scientific discussions at dinner. After Many Waters, they are more inclined to believe in creatures and events beyond the everyday world, but remain less open to them than Meg. In A Swiftly Tilting Planet, they are "embarrassed" that Calvin's mother thinks a rune (in this case a spoken charm or prayer) might help to prevent a nuclear war, and doubt that freak weather conditions are related to either the rune or the nuclear threat.
Both twins occasionally express the pessimistic idea that because of the evil perpetrated by people, it might be just as well if humanity destroys itself entirely. However, they are quick to say that they are "whistling in the dark" when they do this. Their actions and career choices are clearly in the service of making things better, enhancing life rather than destroying it or responding with apathy.
In A House Like a Lotus, Dennys and Sandy (especially Sandy) are very close to their niece Polly O'Keefe.
In A Wrinkle in Time, the twins, age ten, offer to physically defend the rest of the family, and are generally protective of them. However, they understand that their ability to help is limited, and believe that Meg, Charles Wallace and their mother all lack common sense. They maintain a vegetable garden, which is where their siblings and father arrive at the end of the book.
A Wind in the Door, the twins are friendly with a wild super-intelligent snake that lives in a wall at the edge of their property, which they name Louise the Larger after their family doctor, Dr. Louise Colubra. Both insist that Louise is "special", but Meg highly doubted that before her creepy encounter with the Echthroi-Mr. Jenkins. Blajeny says to Meg that they will become "teachers"; but in light of their later careers this identification is not to be considered in a narrow, literal sense. During the crisis the twins work in the garden, not expecting it to help Charles but because they feel the need to do something practical. Afterward, Sandy and Dennys flatly disbelieve Meg's claim that she was inside Charles Wallace's mitochondrion because it's not "realistic", and are "stunned into silence" when Mr. Jenkins agrees with Meg.
In Many Waters, Sandy and Dennys idly type their desire to go "someplace warm" into their father's computer, unaware that an experiment is in progress. They accidentally "tesser" back to the time of Noah with a sonic boom. There they encounter Seraphim, Nephilim, tiny mammoths, hungry manticores and "virtual" unicorns. Separated by illness and circumstance for much of the book, they become less interdependent on one another, and learn that they are not as ordinary as they suppose, but speak the "Old Language" of the stars. Both gain maturity over the course of about a year in Noah's time, each falling in love with Noah's daughter Yalith (and she with them) but not acting on their desires (except for Dennys trying to find a way to save Yalith from the flood). They help to build the Ark before returning home.
In A Swiftly Tilting Planet, Sandy and Dennys are in law and medical schools, respectively, at the age of about 21. They are proud of their parents, which they say keeps their standards high as examples to live up to. Sandy is said to be a history buff. The remains of the vegetable garden is still in the back yard, but the twins no longer are home to care for it on a daily basis.
In A House Like a Lotus, Sandy Murry is Polly O'Keefe's favorite uncle, an anti-corporate lawyer who travels the world with his beautiful Greek wife and colleague, Rhea, fighting greed and the forces of destruction. Sandy has a blond beard, which he wears partly to distinguish himself visually from his twin. He is a longtime friend of Maximiliana "Max" Horne, an artist who becomes Polly's mentor. Dr. Dennys Murry is a neurosurgeon, with a wife named Lucy and a 14-year-old daughter, Kate, named after Dennys' mother. Kate is depicted as pretty and popular, albeit a little spoiled, and currently lives with the O'Keefes while Charles O'Keefe stays with Dennys and Lucy to pursue better educational opportunities. Dennys is a friend of Dr. Ursula "Urs" Heschel, another neurosurgeon and Max's longtime lover. Dennys is said to be primarily a researcher, while Urs is primarily a surgeon. Sandy and Dennys both encourage Polly to be strong and independent, and do not tell Meg and Calvin that Polly is alone in Athens, Greece, Sandy and Rhea having been delayed in joining her there.
Sandy is named after his father, and his nephew, Xan O'Keefe, is named after him. Xan's younger brother Den is named after the other uncle. More generally, Sandy's given name, Alexander, probably refers to Alexander the Great, while Dennys is a short form of Dionysus, the Thracian god of wine. (Dennys is said to be un-Dionysian in temperament, being "both sober and reserved.") Although Polly mentions this derivation of the name Dennys in A House Like a Lotus, there is no definite indication that her uncle's actual given name is Dionysus rather than Dennys. In Many Waters, for example, Sandy acknowledges that his first name is actually Alexander, but in a parallel scene Dennys calls himself only Dennys. (The twins' names are promptly shortened to Den and Sand by the people at Noah's oasis.) The name Dennys is pronounced like the more conventional spelling Dennis, as can be heard in L'Engle's audio recordings of A Wrinkle in Time, A Wind in the Door and A Swiftly Tilting Planet.
As an adult, Sandy Murry combines qualities who had found Douglas Austin and Grandfather Eaton in the Austin family series of books. Like Vicky Austin's Uncle Douglas (Meet the Austins), Polly's Uncle Sandy is a beloved, somewhat adventurous, bearded uncle. Like Grandfather Eaton (A Ring of Endless Light), Sandy is a mentor and confidante to his young, female relative, the book's protagonist. The adult Dennys Murry, in turn, most resembles Vicky's father, Wallace Austin, a doctor who does research in The Young Unicorns.
The twins are shown in several scenes of the 2003 Disney adaption of A Wrinkle in Time, playing foosball, telling their mother about Meg fighting Calvin's brother, leaving for soccer practice and drying dishes. They are portrayed as normal, active children, relatively unaffected by their family's troubles.