Spare a thought for Ed Miliband. At first sight the MP for Doncaster North might not appear to need much sympathy. At the age of just 38 he has a safe Labour seat, a Cabinet position, a sleek flat in Primrose Hill in North London and another home in his constituency. He also possesses dark good looks, an easy manner, a compelling speaking style (I ought probably to admit at this point that he is a friend of mine) and a loyal girlfriend with whom he lives: Justine, an environmental lawyer. Not a lot seems to be going wrong in the younger Miliband brother's life.

But then consider this: there are two men in the world to whom Ed is closest. One is his brother, David, four years his senior. The other is the Prime Minister. And the other day the young Cabinet Secretary opened the newspapers to discover that the two were at war. With the Foreign Secretary being accused by friends of Gordon Brown of acting “disgracefully and disloyally”, and possibly facing demotion, it appeared that Ed was soon to be confronted with the choice nobody would want to make: between supporting his brother or his long-time boss and mentor, the Prime Minister.

The two brothers grew up together in North London, weaned amid the left-wing intellectuals who surrounded their parents, the Marxist historian Ralph Miliband and the academic Marion Kozak, who is still alive and lives near the brothers today. Ralph Miliband, the son of Polish Jews, had fled the Nazis on the last boat to England from Belgium and became one of the most influential socialists of his generation. His sons were encouraged to join the argument with the guests, including their father's close friend Tony Benn, who would pop in for tea.

But it was at the local school, Haverstock, as much as around the dinner table, that their politics was formed. Haverstock was a comprehensive where 64 different languages were spoken and both brothers will cite it unprompted as a crucial influence. What they found there, coming from their comfortable middle-class intellectual circle, was that children more able than them, but from poorer backgrounds, were doing less well, or even leaving school without taking any exams. Both brothers were shocked, and it has propelled them towards the Cabinet ever since. David did not particularly shine at school, coming away with three Bs at A level, and a D in physics - “Ed did better than I did,” he recalls - but won a place at Oxford, where he studied politics, philosophy and economics and got a first before studying for a masters at the Massachusetts Institute of Technology.

At Oxford there was a certain aura about David, because people knew who his parents were. Marked out by a quiet inner confidence, he was known to be leftwing, but was also considered a little geeky and nerdy. He is described by contemporaries as “the opposite of cool” but also extremely nice, and he must have been popular because he was elected president of the JCR. Although coming from the Left, David was already on the side of the modernisers gearing up for the great battle to come for the soul of Labour, and he made enemies because of it - especially when he chose to wear a black tie to a ball at a college that had banished black ties for being elitist.

Ed followed David to Oxford, to the same college, Corpus Christi, to read the same subject, but found himself more interested in activism than academia. “My best four weeks at university were when we had a rent dispute with the college,” he told The Guardian recently. “I wasn't particularly bookish; what really got me going was student activism, and mobilising people. It was quite a hard thing to recognise if you come from an academic family, but, if I'm honest, it's true. Politics always motivated me more than academia.” After college he worked as a speechwriter and researcher for Harriet Harman, and for Gordon Brown, then the Shadow Chancellor. He followed Brown to the Treasury as one of the “two Eds” (the other is Ed Balls) who loyally steered the Chancellor and the Treasury through the ten Blair years.

David, meanwhile, worked his way through think-tanks to become Tony Blair's head of policy at No10, nicknamed “Brains” by Alastair Campbell and working closely with his brother Ed's girlfriend at the time, Liz Lloyd. In dating her for many years, Ed put himself firmly among the Blairite troika of Lloyd, Tim Allan and James Purnell, inseparable Islington flatmates and former schoolmates, originally from Surrey and known as the “Guildford three” (David himself had a brief fling with another aspiring young labourite, Ruth Kelly, now Transport Minister). As the Blair/Brown feud intensified, Ed became an emissary between the Brown and Blair camps. He was known as “the ambassador from planet f***” because he was the only one of the Brown team who didn't tell the Blairites to f*** off.

While nobody at No10 would deal with Ed Balls, who was viewed as an untrustworthy bully, Ed Miliband managed - just - to span the two warring tribes, although there was never any doubt that he was a part of Team Brown.

The two brothers were by now living literally on top of one another, in separate flats in the same house in Primrose Hill. David lived downstairs with his wife Louise, a violinist with the London Symphony Orchestra (the couple have two adopted children, Isaac, 3, and nine-month-old Jacob). Ed lived in the top flat overlooking one of Primrose Hill's most charming squares. After David became an MP and was propelled into the Cabinet, Ed spent a year in the United States, lecturing at Harvard, where he was often refused entry to Boston's bars because he looked so young.

His decision to follow his brother into the House of Commons was hard. Wary of always being in David's shadow, he was uncertain whether to seek a similar frontline role. “There's the classic younger brother, older brother dynamic,” one close friend says, “with the younger brother a bit jealous of the older because he came first, and the older brother not seeing the younger brother as substantial as he is. The jealousy isn't strong, but it's there.”

When I interviewed David for The Times last year (he described the brothers as “probably closer now than we've ever been”) I asked whether there was rivalry between the pair. “I have gentle pride in his achievements,” he replied, before talking about how “incredibly proud” he was of Ed. It did sound like a mother praising her child's gold star at school.

Yet they remain extremely close, albeit with the need to have guarded conversations from time to time. They have a long-standing agreement never to be interviewed together and Ed often introduces himself in speeches as “the other Miliband”. He tells a story about a trade union leader who came up to him at his first conference as a minister, proclaiming: “David, David! I'm really sorry I called you Ed.”

Labour insiders say the younger Miliband is by far the most popular speaker requested by party members and activists. “Ed has a way of connecting with people,” one says. “He lights up a meeting. He inspires them. David is more distant, he doesn't connect.” Ed is more overtly “political” than David, more likely to make a straightforward case for redistribution and fairness, less likely to pontificate about the iPod generation. Where David is theoretical, Ed is personal. When they both spoke at the launch of a biography of their father six years ago (David first), Ed held the room; David didn't. And when Ed appeared on Question Time, an audience member with whom he had argued contacted him afterwards to ask him to go out with her. He is that sort of persuasive.

A friend sees the current clash as potentially liberating for Ed. “Although it's quite clear David is the dominant political force now, over time that will change. Ed would have had to face a choice in five or ten years' time that would have amounted to political fratricide (whether to run for Prime Minister himself). The advantage to Ed of it happening now is that he's not going to have that problem.”

David has told Ed that if he gets to No10, he will judge him on his merits. Ed wonders whether people might think it odd if he were to appoint his little brother as, for instance, Chancellor. The Milibands together have the potential, however, to be Blair-Brown without the flaws, “David as the modernising reformer, Ed as the conscience”, as a friend says.

Ed did not know about the article David wrote for The Guardian this week, which is being read as the launch of a leadership bid by the older Miliband. He loyally says that he believes his brother had no intention of stirring up the excitement that he has. On where he personally stands, he is absolutely clear. Brothers they may be, but Ed is “completely loyal to Gordon”, a close friend says. “If it came to a choice, he would stick with Gordon.”

The more interesting question is, whom will Ed back if it comes to a choice between his brother David and Ed Balls or another candidate? But I'm not sure even Ed would know the answer to that.

Political siblings who keep it in the family
by Alice Fishburn

George W. Bush didn't want to be President when he was growing up, but his little brother Jeb did. He was taller, better looking and harder working, and had the support of the family firm. He thought he had it in the bag. But George won the Texas governorship and the rest is history.

Sibling rivalry has been alive and well since Cain bashed Abel over the head, but political competition undoubtedly takes things up another notch. Bands of brothers (and sisters) roam the political scene: the Kennedys, the Gandhis - and, to go down a few steps, Wendy and Douglas Alexander.

Studying birth order gives us insight into why particular siblings get ahead. Older children score higher on IQ tests and are more likely to have a sense of entitlement: no surprise, then, that George Bush felt, as the elder son, that he was supposed to fill his father's shoes. But younger relatives can often show up their bookish elders in the charisma stakes.

It's a shame that all siblings can't solve their political power-sharing problems with the ease of Lech and Jaroslaw Kaczynski. The identical twins finish each other's sentences and can be told apart only by a few distinctive moles. They also pulled off the unusual coup of being President and Prime Minister of Poland simultaneously.

You only have to look at the dynastic sagas in the Kennedy clan to see that having multiple children is the best way of ensuring a succession.

Joe Kennedy encouraged his four sons to compete against one another from an early age. And in their case, blood proved thicker than anything Washington DC could throw at it. When Joe, the eldest brother, was killed in the Second World War, JFK stepped into his shoes. When JFK fell, Bobby Kennedy picked up the banner. Now the much-lauded Ted Kennedy, a wild child made good, has assumed the responsibilities of his name. Of course, if you can't beat them, you might as well hop on to the bloodline's bandwagon. But the brothers Miliband might want to take note: Stewart Udall made it to the American Cabinet but ended his career managing his younger brother Mo's 1976 presidential campaign. They lost. That must have been a happy family Christmas.

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