The fraternal melodrama has dominated Labour's annual conference ever since votes from trade union members handed Ed the leadership Saturday, though his brother was more popular with party members and lawmakers.
David Miliband left the conference in Manchester, northwest England, two days early, returning to London on Tuesday after his brother gave a keynote speech.
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Contenders have until 5:00pm (1600 GMT) Wednesday to put themselves forward for a place in Ed Miliband's shadow Cabinet, the top team of front-bench opposition spokesmen scrutinising the government.
Ed Miliband appeared to suggest his brother, Labour's foreign affairs spokesman, would step down.
"I certainly don't think you've have heard the last of him," he told BBC radio.
"I know he'll make a big contribution to politics in the future.
"He's just been through a leadership contest he lost, and I think he's got to decide what he wants to do. It's most important that he makes the right decision."
Ed Miliband said he did not believe it would "cast a shadow" over his leadership if his brother did walk away from frontline politics.
David Miliband was Britain's foreign minister for three years from 2007 and impressed in the high-profile role, earning high praise from US Secretary of State Hillary Clinton for his skilled diplomacy.
He repeatedly spurned chances to challenge Gordon Brown openly and replace him as prime minister, and also declined to run for the European Union's foreign affairs post.
Ed Miliband, 40, was asked if he felt he had betrayed his 45-year-old brother by deciding to stand.
"I don't and I don't think he feels that either," he told Sky News television.
Miliband also insisted that Brown had not pushed him into challenging his brother for the leadership.
At the Manchester conference, all eyes were on which lawmakers would stand for the shadow Cabinet.
Labour members of parliament elect 19 people from among their number, including a minimum of six women. Voting opens on October 4 and closes on October 7, with the results due shortly afterwards.
