Born into lives of wealth and privilege, they grow up in palaces, go to elite schools, socialize with stars and travel the world in luxury.
It may sound idyllic, but life as the "spare" heir to the throne isn't always easy.
While their older brother or sister is brought up knowing they are destined for a life regal duty and obligation -- of public engagements, investitures and ribbon cutting ceremonies -- the role of the younger royal is less defined.
As a result they are often torn between duty (and the constant gnawing thought that one day, should something unexpected happen, they may end up ruling) and the life of a "party prince" or princess, holidaying in exotic locations and living it up as a bon viveur.
Royal expert Victoria Arbiter says despite the obvious advantages they enjoy, younger siblings of heirs to the British throne often face a difficult path.
"Throughout the monarchy's 1,000-year history, the role of the 'spare' to the British heir has been a tricky one," she told CNN. "Some have revolutionized entire eras, while others have tarnished the family name."
In the modern era, the "spares" have been Prince Harry -- famously photographed in a string of embarrassing positions: naked during a game of "strip billiards" on a trip to Las Vegas, and wearing a Nazi uniform to a "Bad Taste" party -- Prince Andrew and the Queen's sister, Princess Margaret.
Arbiter says Princess Diana worked hard to make sure Prince Harry didn't feel left out as the second-born child of the heir to the throne.
"When raising Princes William and Harry, Diana was very conscious that William would be well taken care of as the heir, and so she made a concerted effort to include Harry in everything," she says. "And yet he ... has been the subject of intense scrutiny and criticism."
In his younger days, Prince Andrew, the spare to heir apparent Prince Charles, was known for his playboy lifestyle, reportedly romancing a string of eligible young women, earning himself the nickname "Randy Andy."
His reputation was boosted by a stint as a helicopter pilot in the Royal Navy during the Falklands War in the early 1980s, but in more recent years he has again hit the headlines, dubbed "Airmiles Andy" by the British tabloids for his travels around the world while promoting UK trade.
When the current Queen was born almost 90 years ago, there was absolutely no expectation that she would go on to rule -- as the daughter of the "spare," the Duke of York, the plan was for her to enjoy the life of a relatively minor royal.
But when her uncle, Edward VIII abdicated in order to marry divorcee Wallis Simpson, her shy father Albert, known as Bertie to his friends, was thrust into the limelight and became King George VI, a story dramatized in the Oscar-winning movie "The King's Speech."
Later this year, his daughter is expected to break Queen Victoria's record as the longest-reigning British monarch in history.
In eras when childhood mortality rates were far higher than they are today, there was a much greater chance of a "spare" being called on to become King or Queen: The three children of Henry VIII -- himself originally a "spare" rather than an heir to the throne -- all eventually ruled England.
"One child is never enough for a monarch," explains royal historian Kate Williams. "Throughout history, the only way to secure a throne has been with a phalanx of children -- nine for Victoria, 13 for George III."
As they await the imminent arrival of Baby Cambridge #2, it is not yet clear if William and Kate will look to expand their family further, and what that might mean for Prince George's sibling -- or siblings.
But Arbiter, whose father Dickie is a former press secretary to Queen Elizabeth II, says the only certainty to life as a second-born royal is that nothing is certain.
"As history has shown, the British monarchy is anything but predictable -- in two of the last three generations the second-born child has stepped up to the top job and reigned successfully," she says. "Never underestimate the long-lasting potential of a royal 'spare'."
