Ed Miliband and his partner are expecting their second child in November – a brother, he reveals, to one-year-old Daniel. David Miliband is also the father of two young boys.
So will the next Miliband generation create the same sort of political rivalry and psychodrama that are being played out in the current Labour leadership contest, with two brothers dominating the race and rumours of bitter briefing operations on both sides?
In 40 years' time, if Ed Miliband's two sons are both in contention for the leadership, who will their father support – or will he remain neutral, as his mother has done?
The younger Miliband brother laughs. "I will be supporting Diane Abbott, who will at that stage be in her nineties and will be, I am sure, getting on the ballot."
This is clearly a pointed jibe at David's use of his own nomination to ensure Ms Abbott could be a candidate, which has allowed her to pitch to the same left-wing constituency that Ed is appealing to.
But, of his sons, he adds more seriously: "I would advise them not to go into politics, actually."
Mr Miliband, perched on a window seat in his office overlooking the Thames, his head framed by the London Eye, has been speaking passionately for 20 minutes about his vision for the leadership, why the Labour government failed to get re-elected, and why the new coalition is wrong about the deficit.
But when asked to describe the position he is now in, with a likely fight to the bitter end with his elder brother, he hints at the strain it is having on their relationship, how he is having to cope with it changing – although, he hopes, not for ever. Perhaps it is not surprising that he doesn't want history to repeat itself with his two boys.
How does Ed feel when, for example, David slaps him down in public over his opposition to Iraq or when – as happened during the live debate on Newsnight last week – he talks over him?
Mr Miliband says: "It is the 'new normal', in the sense that I never thought I would find myself in the position of – never mind being on national television in a debate with him – but being in a hustings with him.
"It's obviously odd, but it's absolutely handleable. I feel like our relationship is sufficiently close that, win or lose in either case, we will be spending Christmas together, and I think we will look back on it and laugh at what a strange thing it was. Whatever happens, I don't think it will affect our relationship."
The contest so far has been pretty friendly, but last week, amid tensions over Ms Abbott's candidacy, there were rumours that the elder Miliband would not want to serve in his brother's shadow cabinet if he became leader.
Ed says: "I think he will obviously be an outstanding member of anyone's shadow cabinet, and I hope very much he would agree to be in mine, but I am certainly not measuring the curtains."
He promises that his brother will get his second preference vote, just as the shadow Foreign Secretary has pledged that Ed will get his.
Mr Miliband decided he would definitely run for the leadership only in the last few weeks, although he had been tipped as a contender for more than a year. There was speculation before last Christmas that he had even told David he would not stand – a claim that today Ed denies no fewer than nine times. It is "definitely not" true, he says. But it is clear that it is constantly on his mind.
"I love David and I am absolutely determined that we maintain that throughout this contest, and we will do, I am sure. I am going to see more of him than I normally do."
Mr Miliband's other big loyalty lies with Gordon Brown, with whom he worked closely for nearly two decades.
Since the election defeat last month he has spoken to the former prime minister "every so often", adding: "I think he's doing fine. I think it's important in this contest – and I try and do it – that we acknowledge the mistakes the last government made, but we also honour Gordon's contribution. He's an extraordinary person who taught me a huge amount. He taught me toughness, actually, most of all. But I think we owe him a massive debt."
Is Mr Brown following the contest closely or enjoying his new freedom? "I think he's probably got other things to think about."
Ed Miliband's challenge in the race is to distinguish himself not only from his brother but also from the other candidates appealing to the centre-left, Ed Balls and Ms Abbott, as well as from Andy Burnham on the centre-right.
Mr Miliband talks frequently about his "values" – equality, closing the gap between rich and poor, the environment – but it is clear that he needs the full four months of the contest to explain this in more detail, as they are hardly unique to him.
He also wrote the manifesto which helped Labour lose its first election for 18 years, making it difficult for him to disown the Brown government completely. New Labour faltered in government because "we became managers and technocrats", he says, "which seemed to depart from where our values were". This was particularly the case with the banking crisis.
"If old Labour's problem was that it was anti-business, New Labour's problem was it was too close to big business," he says. Instead, Labour must now stand up for small businesses that face "huge vested interests and challenges".
"It [the financial crisis] was a global phenomenon, but we should take our share of the responsibility. Generally, I think in our society people have a sense that the markets are too powerful."
Does this mean that Labour in opposition should shift leftwards, in response to the deficit-cutting Conservative-Liberal Democrat coalition?
Mr Miliband attacks the coalition, calling its members "deficit headbangers". But he singles out the Lib Dems for particular derision, pointing to the withdrawal of the £80m loan to the Sheffield steel castings company Forgemasters, which has caused embarrassment to the local MP and Deputy Prime Minister, Nick Clegg.
"[Mr Clegg] has totally sold out to the Tories – he's revealed himself to be a crypto-Tory. This is exactly what happened in the 1980s under Mrs Thatcher, but this time you have a Liberal Democrat party and a Liberal Democrat leadership which is frankly in cahoots with this agenda.
"If the smoke signals are right about the kind of unfair Budget it is going to be... it will be a total betrayal of Liberal Democrat principles. They will have sold their own party down the river just for the sake of power, and I think people will feel very betrayed by that."
In the days after the election, Mr Miliband was on Labour's coalition negotiating team. There have been a lot of rumours about why the talks broke down, but Mr Miliband, for the first time, says that from the outset the Lib Dem team was totally committed to Tory plans to start cutting the deficit this year, despite during the election tacking more closely to Labour's position of continuing to spend to boost economic growth.
"I haven't talked about this publicly, but... one of the biggest sticking points was around their frankly completely macho position on the question of [cutting] the deficit, saying it needs to be now and it needs to be faster. They were completely cavalier about that."
He claims that two Lib Dem negotiators in particular – Chris Huhne, now Secretary of State for Energy and Climate Change, and David Laws, who resigned as Chief Secretary to the Treasury earlier this month – were "very vociferous" about cutting the deficit now.
As the Energy Secretary who led Britain's delegation at last year's Copenhagen climate talks, and now as shadow Energy Secretary, Mr Miliband says the BP oil crisis – while a "terrible tragedy" – provides an opportunity for us to "get off oil", which he says is the "biggest challenge the world faces".
"If you ask what Daniel is going to be saying to me in 40 years' time... it would be: 'What did you do on climate change? What did you do on the post-oil economy? Did you make decisions to change the course of events?'
"Out of the crisis has to come a change. The world has hiccupped a number of times on the way to this transition. I know from Copenhagen it is incredibly difficult, and therefore you've got to use this moment to say, this is when the world's got to realise we've got to get off oil."
As Diane Abbott has been at pains to point out, her male rivals for the leadership are all obsessed with football. Ed Miliband is a Leeds United fan. So what leadership advice does he have for Fabio Capello, as England battle through their World Cup campaign?
He offers imagination, inspiration and loyalty – Capello should have stuck with goalkeeper Robert Green, for example. Clearly, loyalty matters to the younger Miliband, but choosing between goalkeepers is easy; deciding to run against your brother for the job he's always wanted is not.
December 1969 Born in London to Polish Jewish immigrants and left-wing intellectuals Ralph Miliband and Marion Kozak; younger brother to David.
1980-87 Attends Haverstock Comprehensive school, north London
1987 Spends a summer doing work experience for Tony Benn
1987-90 Studies politics, philosophy and economics at Corpus Christi, Oxford
1994 Begins working for the shadow Chancellor, Gordon Brown
May 1997 After Labour's first election victory, becomes Brown's speechwriter
2003-04 Takes year's sabbatical to study at Harvard
May 2005 Elected Labour MP for Doncaster North
May 2006 Appointed by Tony Blair as a junior minister in the Cabinet Office
June 2007 Appointed to Gordon Brown's first Cabinet alongside David Miliband – they become the first brothers in the Cabinet since 1938
October 2008 Made Secretary of State for Energy and Climate Change
May 2009 Has a son, Daniel, with his partner, Justine Thornton
2009 Put in charge of writing Labour's general election manifesto
December 2009 Leads Britain's delegation to Copenhagen climate change talks
May 2010 Stands for Labour leadership