Martyn Key (68) is one of two sons from an earlier marriage of John Key's father, George. The other lives in Canada.
Eugene Bingham, of The New Zealand Herald, tracked down Martyn Key in an investigation into the life of the man who would be Prime Minister, published in July.
Martyn Key was surprised to learn he had a famous youn-ger brother.
One of John Key's sisters had been in touch with him 30 years be-fore, but other than that there had been no contact.
"It took Eugene Bingham to play the dating game to get us together," the Prime Minister said yesterday.
"But he [Martyn] has emailed me and I have emailed him a couple of times.
"It'll be great. I am really looking forward to it," he said shortly before leaving Peru where he has been at the Apec leaders' summit.
Mr Key and Mr Brown are expected to compare notes over the G20's response to the financial crisis, which Mr Brown attended, and the Apec summit.
The meeting is scheduled for the day after Mr Brown and Chancellor of the Exchequer Alistair Darling are expected to unveil a stimulus package that will cut VAT from 17.5% to 15%.
Mr Key expected the discussion would be dominated by the financial crisis but go further.
"This is a relationship of considerable proportion, obviously, and spans many areas and I imagine we will touch on them."
Asked if he had given thought to New Zealand's response should Nato request a bigger contribution in Afghanistan, Mr Key said he had no briefing on that but he expected to be asked at some point.
In a one-day visit, Mr Key will also meet Conservative Party leader David Cameron and London Mayor Boris Johnson, who will attend the launch of the giant promotional rugby ball for the 2011 Rugby World Cup near the Tower of London.
Mr Key was sworn in as prime minister on Wednesday last week and left for Apec on Thursday.
A firm commitment from Pacific-rim leaders to fast-track global free-trade talks was a welcome boost for the faltering world economy, Mr Key said at the end of the talks.
He was one of 21 leaders at the summit to sign a declaration pledging to act "quickly and decisively" to overcome the global financial crisis within 18 months.
Mr Key said he was satisfied with the final communique, which included an agreement by leaders to send their foreign ministers to Geneva next month in a bid to re-start the stalled Doha Round of World Trade Organisation talks.
"I think it reflects the desire to show real leadership."
Mr Key said his personal highlight was meeting United States President George W. Bush, and China's President Hu Jintao.
