Lilibet was 10 and Margaret was six when a twist of fate turned these minor aristocrats into the heir and the spare.
By rank, the sisters should have led a luxurious life on the royal periphery, comparable to that of princesses Beatrice and Eugenie in the present day.
But on December 11, 1936, their childless uncle abdicated the throne and hurtled them towards their new destiny.
Lilibet was to become Queen Elizabeth II, Britain's longest-reigning monarch and one of the most significant women in history.
Margaret, a girl with a sparkling wit and a craving for the spotlight, would spend the rest of her life in her beloved sister's shadow.
Their differences could have driven them apart. Like so many royal siblings throughout history, the crown could have come between them.
But these women shared a "primal bond", according to biographer Andrew Morton.
"Margaret was her sister's alter ego," he wrote in Elizabeth and Margaret: The Intimate World of the Windsor Sisters.
"Throughout their lives Elizabeth and Margaret butted heads — the sensible, older sister matched with the mischievous, wilful little sister."
But their devotion would be tested by a doomed romance, a looming constitutional crisis and a string of scandals.
Two very different women
The House of Windsor has a history of producing a dutiful, conservative heir followed by a charming, slightly dangerous, sibling.
"Charles the patient versus Andrew the foolish. William the cautious matched against reckless Harry," Andrew Morton said.
Princess Margaret said she and her sister were often reduced to archetypes fit for a fairytale.
"When there are two sisters, and one is the Queen, who must be the source of all honour and all that is good, the other must be the focus of the most creative malice, the evil sister," she said.
Despite an uncanny physical resemblance, they were extraordinarily different people.
Elizabeth's morning followed a strict routine fit for a monarch: Awoken by bagpipes, she would take tea alone before settling down to read her correspondence and the government documents requiring her approval.
By contrast, her sister rose at 9:00am, chain-smoked in bed and took a bath run by her ladies-in-waiting.
After having her hair and make-up done, she made her way downstairs for a "vodka pick-me-up" at noon.
She would then join her mother for a "four-course lunch served in an informal manner from silver dishes with half a bottle of wine per person, plus fruit and half a dozen different varieties of native and continental cheeses."
The Queen spent decades mastering the art of small talk with thousands of people at garden parties and state dinners.
But her little sister was better known for wild house parties during which she sang, played piano, danced, gossiped, and downed expensive Scotch.
"Disobedience is my joy," she once told French filmmaker Jean Cocteau.
She smoked so much, according to her biographer Craig Brown, that she would glue a matchbook to her whisky tumbler "so that she could strike matches while drinking".
She mixed with a crowd of British celebrities and bohemians, who enjoyed having a princess in their midst.
But they were also forced to endure her sharp tongue.
'The house guest from hell'
After reeling people in with her charm, Margaret was notorious for suddenly hurling insults without warning.
"Well you don't look like a movie star," she said to Grace Kelly when they met in the 1950s.
She informed Elizabeth Taylor her engagement ring was "vulgar" and told the model Twiggy her nickname was "unfortunate".
She also exploited royal protocol, earning herself the title of "the house guest from hell" from those who entertained her.
She often showed up late to dinner parties, knowing the meal could not start until she had arrived.
"Dinner was at 8:30pm, and at 8:30pm Princess Margaret's hairdresser arrived, so we waited for hours while he concocted a ghastly coiffure," English novelist Nancy Mitford recalled of a party in Paris in 1959.
Her short temper and obsession with rank was an attempt "to control what she could", according to Helena Bonham Carter, who knew the princess and played her in the Netflix series, The Crown.
"If you have very little control in your life, you often end up controlling what little that you can," the actress said.
But perhaps the biggest difference between Margaret and her sister was their taste in men.
Margaret's tragic love life nearly tore apart her relationship with the Queen, and threatened the future of the monarchy.
A forbidden romance
After the love affair between King Edward and divorcee Wallis Simpson triggered a constitutional crisis, the royal family were keen to avoid any more romantic scandals.
And so when Princess Margaret announced at 22 that she had fallen in love with Peter Townsend, a divorced royal equerry 16 years her senior, they were swift to cover up the relationship.
"She was a girl of unusual, intense beauty confined as it was in her short slender figure and centred about large eyes, generous, sensitive lips and a complexion as smooth as a peach," Group Captain Townsend wrote in his memoir.
"But what ultimately made Princess Margaret so attractive and lovable was that behind the dazzling facade, the apparent self-assurance, you could find, if you looked for it, a rare softness and sincerity."
Hardly any photos of the couple exist, and palace courtiers continued to spruik Margaret as a single, glamorous and eligible young princess.
But on the day of her sister's coronation, a tiny gesture gave the secret couple away.
Waiting at Westminster Abbey for her carriage back to Buckingham Palace, Margaret was spotted by the media brushing a piece of fluff from Peter Townsend's uniform.
The intimacy of the moment was confirmation enough for the British press and they printed a story about their romance two weeks after the coronation.
While the new Queen had asked the couple to give her a year to settle in before she would grant them permission to marry, the tabloid story sent her courtiers into a panic.
They ordered Group Captain Townsend to move to Brussels to serve as air attaché at the British embassy.
Princess Margaret was on tour in Rhodesia at the time.
When she returned, the love of her life was gone.
Margaret must choose: status or love
For the next three years, the question of marriage consumed Margaret and Peter, as well as Buckingham Palace.
The Queen, though nervous about the disapproval of her parliament and the Church of England, supported her beloved Margaret.
"Her Majesty would not wish to stand in the way of her sister's happiness," then-prime minister Anthony Eden wrote in a letter to the Commonwealth.
Together the Queen and Mr Eden formulated a plan. Princess Margaret could marry her love while keeping her royal title and allowance.
She could even continue with royal duties if the public approved, which was likely, given that by 1955 people regularly shouted to her: "Go on, Marg, do what you want!"
But there was a catch. She would have to renounce her rights of succession and those of her children, and marry in a civil ceremony.
Three days after the Queen and her prime minister finalised the plan, Margaret broke up with Peter Townsend forever.
"Mindful of the Church's teaching that Christian marriage is indissoluble, and conscious of my duty to the Commonwealth, I have decided to put these considerations before any others," she said in a statement.
"I am deeply grateful for the concern of all those who have constantly prayed for my happiness."
Whether she was bullied by courtiers, loyal to the crown, or falling out of love, Margaret made her choice: To remain by her sister's side.
Princess Margaret meets her match
When they parted, Margaret and Peter Townsend made a pact to never marry anyone else.
Four years later, Group Captain Townsend proposed to a Belgian woman 25 years his junior.
Betrayed and heartbroken, the princess knew it was time to move on with her life.
A year after the Townsend wedding, she walked into Westminster Abbey surrounded by 2,000 guests, and married Antony Armstrong-Jones.
The magazine photographer was the first commoner to wed a king's daughter in 400 years, and the Queen gave her new brother-in-law the title of Earl of Snowdon.
"The [royal family] liked him very much — Tony had great charm, very good manners and he knew exactly how to behave," Anne de Courcy wrote in her biography of the earl.
"He felt devotion to the royal family, to the Queen, who he admired immensely."
Her marriage ushered in an era of stability for Princess Margaret, who gave birth to two children and stayed close to her mother and sister.
The Queen had a direct phone line installed from Buckingham Palace to Margaret's apartment in Kensington Palace so they could chat whenever they wanted.
The princess and Lord Snowdon were at the centre of London high society in the 1960s, mixing with celebrities and intellectuals.
But the relationship slowly started to unravel as both husband and wife engaged in affairs.
By the early 1970s, the couple were so at odds that Lord Snowdon would hide abusive messages in Margaret's belongings.
"He took to leaving nasty notes on her desk, including one headed '24 Reasons Why I Hate You,' which particularly upset her," biographer Craig Brown said.
"On [another occasion] a note [was] tucked into her bedside book, saying simply, 'I hate you.'"
In 1976, with her marriage all but over, Princess Margaret's romantic life triggered another scandal.
But this time she would not sacrifice love for the crown.
Margaret's tragic final years
Roddy Llewellyn was a 25-year-old gardener partying in Scotland when he caught the eye of a 43-year-old princess in the midst of a secret marriage breakdown.
They kept their affair under wraps for several years, but a paparazzo with a long lens camera eventually spotted them on a beach in Mustique.
"I didn't think about the consequences of such a high-profile affair," Mr Llewellyn later said of their relationship.
"I was just following my heart."
The British tabloids were aghast at the relationship between a married older princess and the man they referred to as her "boy toy".
Members of parliament called Margaret a "floozy" and a "parasite". But despite pressure from her sister and the government, Margaret refused to give Roddy up.
And in 1978, she became the first royal since King Henry VIII to divorce, officially ending her marriage to Lord Snowdon.
The rebel princess reduced the number of public appearances she made and spent more time in Mustique with her lover.
However, her second era of relative stability did not last long.
In 1981, after eight years together, Roddy Llewellyn and Margaret broke up.
She would spend the rest of her life alone, holidaying with friends on tropical islands and making an occasional appearance with her sister.
While the Windsor women are known for robust health and longevity, a lifetime of cocktails and cigarettes began to take their toil on Margaret when she reached her 50s.
After years of ill health, she died in her sleep from a stroke in 2002.
At the funeral, Elizabeth, known for her stoicism through war and tragedy, wiped tears from her face with a handkerchief.
It was one of the few times the Queen ever cried in public.
Lady Anne Glenconner, who introduced Princess Margaret to Roddy Llewellyn, was at the funeral, and tried to avoid an encounter with the Queen.
"It was difficult for the Queen and I felt rather guilty always, having introduced Roddy to Princess Margaret," she said.
But the Queen spotted her after the service and asked to speak to her.
"She said, ‘I’d just like to say, Anne, it was rather difficult at moments, but I thank you so much [for] introducing her to Roddy because he made her really happy,'" Lady Anne said.
Margaret may have been called Elizabeth's alter ego and her "evil" twin.
But really, she was a woman with no purpose but to be a spare part in a gilded machine.
She spent her life seeking love and connection with men she could not have, or worse, men who let her down.
In the end, the most enduring and significant relationship Margaret had was the one she shared with her sister.
