Imagine your younger brother had just been humiliatingly defeated in a national election. How would you respond? Perhaps with a phone call to offer commiserations? Maybe a hug, too?

You probably wouldn’t give an interview to the BBC four days later in which you claimed the electorate “didn’t like what was being offered” because your kid brother looked like he was “moving backwards”. You might not feel the need to then quip to a group of Harvard students that you were in the “backroom … when Labour in the UK figured out how to win elections rather than lose them”. You maybe would avoid telling CNN’s Christiane Amanpour that Labour, under your little brother, “turned the page backwards rather than … forwards”. Perhaps you would also keep to yourself the fact that Labour’s defeat had “confirmed” all your “worst fears”, rather than admitting to the Times that you had been worrying for several years that the party was “courting disaster”.

Then again, you’re not David Miliband. The former foreign secretary has spent the past five weeks compensating for five years of studious silence. His not-so-coded message since Labour’s election debacle can be summed up in five words: They. Picked. The. Wrong. Brother.

Ed Miliband’s allies are, perhaps understandably, apoplectic. “David has demeaned himself,” a close friend of the former Labour leader tells me. Equally understandably, David’s allies believe the party’s catastrophic defeat has liberated him from any fraternal obligations.

It is easy to forget that these two brothers, in the words of a senior Labour figure who knows them well, were once “profoundly close”, with a “real bond” between them. The children of Holocaust survivors, they grew up in a small, tight-knit family. Ed, four years younger, looked up to David; he followed in his elder brother’s footsteps – first to Oxford university to read PPE, then into the Labour party as a special adviser and, finally, into parliament and then government. For a while they lived in adjoining flats; upon being elected to parliament, in 2005, backbencher Ed would occasionally even share a ride back to Primrose Hill in the back of big brother David’s ministerial car. The younger Miliband was best man at his elder brother’s wedding to violinist Louise Shackleton in 1998.

It’s now clear that David has yet to forgive his little brother for standing against him and (narrowly) defeating him

Today, Louise and Ed’s wife Justine don’t talk. David wasn’t asked to be best man at Ed’s wedding in 2011. The days of regular Sunday lunches and Christmas get-togethers, between the brothers, their wives and their mother Marion, are long gone. David has been living 3,000 miles away in New York for the past two years.

The relationship between these two siblings irrevocably changed the day Ed decided he wanted to be leader of the Labour party, too. Both brothers, raised in an ultra-political household, had wanted to be prime minister since they were teenagers, a friend of their late father Ralph once revealed to me. Both believed – and still believe – their vision of social democracy was the correct one. But most siblings wouldn’t, of course, end up running against each other. Perhaps they underestimated, Ed in particular, how difficult it would be to go up against their own flesh and blood. Ultimately, political ambition trumped fraternal allegiance. “I don’t understand the dynamics between the two of them,” a former Labour cabinet minister and close friend of the Milibands has observed. “Have you met anyone who can properly explain it to you? I just simply don’t understand it.”

Haunted by constant references to Cain and Abel (or is it Romulus and Remus?), the younger Miliband has spent the past few years trying to pretend that his relationship with his big brother is “on the mend” but it’s now abundantly clear that David has yet to forgive his little brother for standing against him and (narrowly) defeating him. The man once described by Tony Blair as the “Wayne Rooney of my cabinet” had dreamt of becoming Labour leader and then prime minister and yet his dream was snatched away by, of all people, his younger – and perhaps crucially – more leftwing brother. Much is made of “the so-called psychodrama of fratricide” says the friend of Ed, but David’s recent comments “reveal a political chasm between him and Ed” and are, thus, a reminder that the younger Miliband’s “decision to stand for the leadership was about the ideological difference between them”.

Was Ed too leftwing, though, for the British public? If ideology is the issue, claim the Blairites, the centrist David could have won the election for Labour. Really? How would deficit hawk David have saved Labour from meltdown in anti-austerity Scotland (where, incidentally, the two co-chairs of his own leadership campaign, Jim Murphy and Douglas Alexander, both lost their seats)?

How would the pro-EU, pro-immigration David have fended off the threat from Ukip? And is the elder Miliband – who referred to the party’s “valence” and “composite qualities” in his Times interview on Wednesday – any less geekish or wonkish than the younger Miliband? Can the banana-wielding former foreign secretary be said to be any less gauche or gaffe-prone than the bacon sandwich-eating former leader of the opposition? Then there is the aloofness: “David would have split the Labour movement with his remote personality,” says a member of the shadow cabinet.

Despite their myriad flaws, David and Ed Miliband undoubtedly remain two of the most talented Labour politicians of their generation. Thus theirs is both a political and a personal tragedy: one brother tried and failed to become prime minister while the other will never get the opportunity to try.

This past week, allies of David have been briefing that their man can still become Labour leader after 2020, if Ed’s successor fails at the next election. This is nothing short of a delusion. As Labour’s former deputy leader, John Prescott, rightly said on Thursday: “The Miliband period has gone. We are not looking to a period where [David] emerges as another Miliband interpretation. I don’t think that’s possible.”

He’s right. So was it all worth it? The political fallout? David’ self-imposed exile in the US? The seemingly permanent rupture in the once-close Miliband clan? Consider the verdict of their mother. A few weeks before the result of the Labour leadership contest was announced in September 2010, Marion bumped into an old family friend on a trip to New York. “You must be proud of them,” he said to her, referring to the fact that Marion’s two sons were the frontrunners. “Proud?” she replied. “One is not proud of adults, one is proud of children. And if they’d asked my advice, I would have told them not to get into this ridiculous game.”