Rilba Jones, 79, a retired health visitor from Hull, and Mary Brand, 73, from London, Ontario, 3,500 miles away, met after uploading their DNA to genealogy website ancestry.com.
As well as a striking family resemblance, they found they have a huge amount in common, with their newly enlarged family including writers and campaigners on both sides.
For Mrs Jones it has been a “life-changing” experience after finally confirming the truth of her birth, which she has always suspected.
As a youngster Mrs Jones was told her father was Flight Lieutenant Blair White, who was shot down and killed in the invasion of Sicily in 1943, when she was just two.
But aged 14 and put in a back bedroom as a punishment, she discovered an account which had been locked away in a trunk telling of her mother’s pregnancy and subsequent efforts by relatives to find a “suitable” husband.
However her mother denied it when confronted and the subject was never brought up again. Her mother went on to remarry for a second time.
Last Christmas she was given an ancestry.com gift by her daughters Charlotte and Harriet.
Mrs Jones, who had always been told she had Danish looks, thought she might find out more about her Scandinavian ancestry - but got far more than she bargained for.
Meanwhile half sister Mary, a retired primary school teacher, was also on the search.
A keen investigator of her own family history, she had already uploaded her own DNA to ancestry.com and Mrs Jones’s details came up as a possible “first cousin”.
Mrs Brand said: “My daughter Googled her, and we found her right away. She looks very much like my older sister Elizabeth and my younger brother John.”
She added: “We had no idea that there was a half sister in England. I’m positive he had no idea either.”
Initially Mrs Jones suspected her sister was an Internet scammer, when she tried to get in touch, but then their daughters got in contact via email.
The sisters met in Hull in October and Mrs Jones immediately felt a sense of belonging: “I’ve wondered all my life about my identity and this has been life changing.
"Mary said: ‘You’re smart - like our Dad'.
“In my ‘birth family’ I never felt I fitted in - they were not really my thing. With Mary’s family I really did. I’ve had a very warm welcome and it has made me feel much more confident.”
Her real father John Norman Harris, who left Canada to join the RAF in 1937, probably met Mrs Jones’s mother when he was recuperating from plane crash injuries at Torbay Hospital in Torquay in 1939.
Mrs Jones was born in September 1940 in Torquay nine months later.
She said: "By that time my mother had got married to Flt Lieut White. She never let her own family know. My cousins are all delighted that it's finally come out.
"My mother died nine years ago. She was very attractive and took a lot of trouble over herself, but always wanted to be respectable."
Flight Lieutenant Harris's plane was shot down in 1942 during a raid on Hamburg and he spent the rest of the war at the Luftwaffe-run camp Stalag Luft III, which held captured Western Allied air force personnel.
In 1943 prisoners constructed a gymnastic vaulting horse to disguise men, tools and containers of of soil, while a tunnel was dug, allowing three men to escape to safety.
“While we were vaulting over the horse and playing follow-my-leader, a man was underneath working in grim earnest”, Mr Harris later recalled.
Mr Harris’s role in the break-out was re-enacted in the 1950 film The Wooden Horse.
Son John R Harris said his father was depicted as the “klutzy guy” who knocked the horse over on days they weren’t digging so the Germans did not get suspicious.
He said: “That was my Dad, who hurt his back in the plane crash. Every time he did make it over, guards in the towers would jeer derisively.”
He was apparently scheduled to be seventh man out, as he spoke passable German, but in the event only three made it.
After the War he returned to Canada with Aileen Dunkeld and went on to have four children.
A talented writer, he was best-known for his mystery novel The Weird World of Wes Beattie, published shortly before his death in 1964.