Labour contenders: Ed Miliband
Perhaps we should have guessed what was coming in 2005, when delegates at Labour's annual conference were seen wearing "My favourite Miliband" badges, with pictures of either Ed or David on them.
Yet the sheer strangeness of two brothers battling it out for the leadership of a major political party has still taken some getting used to.
Ed - the younger and less well-known Miliband - refers to it nonchalantly as the "new normal" and both men have insisted they will not let it ruin what they say is a close relationship.
But with Ed emerging as his brother's chief rival for the Labour leadership - by early July he had gained more nominations from local constituency parties than David - it is only natural that tensions will start to emerge.
Particularly as Ed's supporters talk of him as the more "human" of the two, a better listener who is more in tune with the core Labour values.
He has been praised by some in the Labour Party for the determination, even ruthlessness, he has shown in challenging his brother for the top job, particularly when it was assumed by many that David was the natural heir to Tony Blair.
Others have warned of hubris and speculated about whether he has the experience to be truly considered a prime-minister-in-waiting.
Eyebrows were raised during the early stages of the campaign when Ed arrived at hustings with his own band of placard-waving supporters.
The two Miliband brothers grew up in North London. The sons of one of Britain's leading Marxist intellectuals, Ralph Miliband, they both went to the same comprehensive school and Oxford university, before beginning a rapid ascent through the Labour ranks, albeit on opposite side of the Blair/Brown divide.
A self-confessed Maths "geek", Ed's number-crunching skills were highly prized by Gordon Brown at the Treasury but he also gained a reputation as something of a diplomat, whose skill at defusing rows was reportedly much in demand in the never-ending war of attrition between Brownites and Blairites.
It is said that Ed would often be despatched from the Brown camp to make peace with Downing Street, where David worked.
"I was the one who tried to bridge some of the nonsense that there was," is how he now describes his role.
But he baulks at the usual description of himself as a "Brownite", claiming to be one of the least "tribal" of MPs.
Like his brother, he belongs to the generation of Labour politicians who, until recently, had known nothing but power, having become an MP for the safe seat of Doncaster North in 2005.
At 40, he is four years younger than David but lives in the same fashionable district of North London, Primrose Hill, as him, with partner Justine and their young son.
Although essentially cast from the same centrist, New Labour mould, he has positioned himself firmly to the left of his brother during the leadership race, campaigning for a "living wage" higher than the minimum wage and a High Pay Commission to control top salaries.
He has won support from the left by calling for the retention of the 50p tax rate and opposing a third runway at Heathrow and he was widely praised by green activists during his time as climate change secretary.
Also, despite being credited with writing it, he has been far more critical than David of Labour's 2010 election manifesto, telling his brother at one hustings: "How can you possibly say you're going to stand on every aspect of our manifesto? We lost the election."
He has spoken of a four or five year plan to remodel the British economy by tackling the gap between rich and poor and creating a broader industrial base.
In contrast to David, he has also backed a graduate tax to allow tuition fees to be scrapped.
The brothers have also disagreed on Iraq - with Ed calling the 2003 invasion a "tragic error" and saying he would have voted to give weapons inspectors more time had he been an MP at the time.
Ed has also taken a few well-aimed swipes at the other leadership contenders - in particular his former Treasury colleague Ed Balls.
He told a Westminster press lunch he had no need to brief against colleagues when he shared an office with the "forces of hell" - a not so thinly veiled reference to the shadow education secretary.
Responding to accusations that he was simply saying what Labour activists wanted to hear with his appeals to the left wing, Ed told the same event "we cannot define ourselves in opposition to our party" and argued that the "centre ground should be shaped from the left".
Like the other contenders, he has spoken of the need to move beyond the Blair/Brown era, calling for an end to the "factionalism and psychodramas" of Labour's past.
But some in the party will surely be wondering if a whole new set of factions will emerge in the aftermath of the leadership contest, particularly if Ed succeeds in upsetting what many had assumed to be the natural order by beating David to the crown.