Prince William and Prince Harry have two 'secret' siblings the world totally forgot about.

Many of us who are familiar with the British royal family (and especially those among us who binge-watched every available episode of The Crown in a single weekend…) would be able to draw a pretty accurate, quick family tree.

There’s Queen Elizabeth II and Prince Philip, of course, along with their children Princes Charles, Edward and Andrew and Princess Anne.

Prince Harry and Prince William are the result of Prince Charles’ marriage to Diana. And then are their respective partners, Meghan Markle and Kate Middleton, who are often dominating the headlines with their fashion choices, and William’s cute-as-a-button children.

LISTEN: The Crown’s season two just solved a major issue about scones. Post continues after audio…

But there’s a branch that is often forgotten from this royal family tree: the children of Prince Charles’ second wife, Camilla, Duchess of Cornwall.

Yep, Prince Harry and Prince William officially have two step-siblings: a 43-year-old brother, Tom Parker Bowles, and a sister, Laura Lopes, 40.

Interestingly enough Prince Charles was actually named Tom's godfather when he was born in 1974.

Tom is a successful food writer, having published five cookbooks, and has appeared as a judge on a number of food-related television shows.

He married Sara Buys, a fashion editor at Harpers & Queen (now known as Harper's Bazaar UK), in 2005 and the pair have two children: 10-year-old daughter Lola and Freddy, who turns eight at the end of this month.

Wills and Harry's step-sister Laura is a little more elusive than big-brother Tom, and has appeared to shy away from the spotlight that inevitably comes with being connected in any way, shape or form to the British royal family.

Here's what we do know: when she married her husband Harry Lopes in 2006, she famously donned a wedding dress from the same designer who dressed mother Camilla for her wedding to Prince Charles just a year earlier.

Her new stepsiblings, Prince Harry and Prince William (who brought then-girlfriend Kate Middleton as a date) were in attendance when she tied the knot.

In early 2008 Harry and Laura welcomed a daughter, Eliza, and on 30 December 2009, the couple welcomed twin boys named Gus and Louis.

And while Laura's face may not be instantly recognisable, her daughter's is: Eliza, then just three years old, stole the hearts of royal watchers across the world when she appeared as a junior bridesmaid at the 2011 wedding of Prince William and Kate Middleton.

Despite being largely 'hidden' from the public view, it's believed Tom and Laura will both be in attendance when Prince Harry and Meghan Markle tie the knot in May.

So get your binoculars ready for some 'secret sibling' spotting.

LISTEN: Mamamia's TV and pop culture podcast The Binge deep dives on The Crown season 2: