Sonia Purnell is the author of 'Boris: A Tale of Blond Ambition'. Her views do not necessarily reflect those of ITV News.
Boris and Jo Johnson share the family's trademark ultra-competitiveness, but in many ways they are very different people.
A dinner party guest who once sat next to Jo told me that the conversation was 'excruciatingly' serious all evening. He doesn't do small talk or jokes.
He sits on his own in the canteen, he likes his own space and, unlike his brother, he does not empathise with people, which can cause offence at times.
But in many ways, he is better positioned than his older brother to one day lead the Conservative Party.
It is wrong to underestimate Jo - he is a man in a hurry but he is also a man with time on his side.
He is now in a pivotal government role; it is his time to shine and do not forget he has only been an MP for three years.
Jo is extremely bright, self confident and driven, he is seen as close to Chancellor George Osborne, which must have caused awkward moments at family gatherings.
Just after Boris was elected as Mayor of London in 2008, there were a lot of rumours that Jo would become head of policy for his older brother.
But apparently there was unease within the party over this plan, with concerns that the mayorship could become Boris Inc.
The London Mayor's youngest brother attended Eton before going to Oxford University, where he was also a member of the Bullingdon Club.
He is highly regarded in the party and interestingly, is married to a Guardian journalist, who is politically to the left of him.
Although he is close to Boris, he has a stronger relationship with his sister Rachel, who mothered him when they were growing up.
His mother Charlotte is witty, bright and a very good painter, while his father Stanley is competitive and driven.
The children had a tough childhood. At the ages 11 and 10, Boris and Rachel both had to commute alone to boarding school on a ferry and they were not encouraged to have friends.
Rachel once said they were taught to be self-sufficient and self-reliant.
Jo grew up in an ultra-competitive environment, where dinner time was all about who was the cleverest or who could run the fastest.
The next competition in the Johnson household could be who can lead the country.
Sonia Purnell is the author of 'Boris: A Tale of Blond Ambition.'**