Labour's first family, the Miliband brothers, stole the attention of Westminster watchers today as Ed made his first speech in the House of Commons since his general election defeat and David's UK comeback was announced.

The relationship between the two turned sour after Ed decided to run against David, the elder Miliband, for the labour leadership in 2010. The Doncaster MP, with the support of the trade unions, beat the former foreign secretary and took Labour leftwards.

Meanwhile, David reportedly turned down his brother's offer to become shadow chancellor, quit the Commons in 2013 and moved across the Atlantic to take a prestigious charity job as the head of International Rescue Committee.

The Blairite remained relatively quiet when it came to British politics while in America, focusing his attentions on his charity work. But David's personal opinions went public when he criticised his brother's premiership in the wake of Labour's worst general election result since 1983.

The elder Miliband, speaking in New York, told the BBC that there was "absolutely no point in blaming the electorate" for the shock Tory majority and Labour's misery.

David didn't stop there, he turned the screw. Ed and Gordon Brown had "allowed themselves to be portrayed as moving backwards from the principles of aspiration and inclusion that are the absolute heart of any successful progressive political project," he argued.

He then ruled himself out from the Labour leadership race, triggered after his brother's top flight demise. This fact makes his visit to London's Royal Albert Hall on 6 October all the more intriguing.

He will give a keynote speech at the Institute of Directors' Annual Convention in front of more than 2,000 business leaders. We are told that he will speak on the theme of "Business Beyond Boundaries", and is expected to talk about how humanitarian crises affect business.

The timing will not be lost on Labour's leadership contenders as the talk will come less than a month after Ed's successor is announced on 25 September.

The younger Miliband has kept schtum about the race since he stepped down after the election, when he took "full responsibility" for the result. Ed has been spotted near the green benches after a family trip to Ibiza but today saw his first speech as part of a debate on the Queen's Speech.

The former energy secretary urged George Osborne to do more about in-work-poverty and pressed the Chancellor, with numerous references to Benjamin Disraeli's "One Nation" philosophy, over the issue of inequality.

"I believe this is an issue for both right and left. What's changed in the debate about inequality is that, internationally and across the political spectrum, there is growing recognition that these gaps are not just bad for the poor – as we used to believe – but bad for all of us," he said.

"More [unequal] societies are societies that tend to use the talent of fewer people and suffer as a result...This issue will not go away and it needs to be confronted."

Ed, a former energy secretary and party leader, now sits on Labour's backbenches.