There's a fine balancing act to being the second born in a monarchy. Lightheartedly deemed 'the spare to the heir,' it's important that they not only support their elder sibling wholeheartedly but also carve out their own path in life.
These days, there's little likelihood that you will be needed to take the throne yourself. But you will always have an invaluable role to play, as someone who has known the sovereign from their earliest years and understands them probably better than anyone else.
No one illustrated the challenges of the position more acutely than the Queen's younger sister, the late Princess Margaret. The siblings were as close as could be during childhood, and their father King George's death, and Elizabeth's accession, only strengthened their bond.
Those who knew Margaret emphasize her sense of fun and her total loyalty, both of which must have made her company a joy to Elizabeth as she took on the serious and sometimes lonely role of monarch. Margaret's devotion to her sister was such that it played a part in her painful decision to give up her first love, Group Captain Peter Townsend, who, as a divorcé, was not considered a suitable royal match. Right up until her death, she and Elizabeth spoke every day.
In the younger generation, Prince William's brother Prince Harry has won countless fans for the way he's handled his position as second born. Alongside an exuberant personality, he has developed a well-respected public persona.
Harry has adopted a string of worthy causes, from the fight against landmines to the plight of war veterans and disaster victims. His little niece, Princess Charlotte, will find he can offer her plenty of helpful advice as she grows older.
Naturally, she will also look to all the other young second siblings in royal houses across Europe and beyond, from Princess Alexia of the Netherlands to Prince Hashem of Jordan.
Click on the photos above to look at the lives and characters of these youngsters, who provide an often unsung, but sterling supporting act.