Sarah Kemp, 42, travelled from her Edinburgh home to London to meet George Bentley, 47, after exchanging emails for three months.
They only realised they were related while chatting about their childhood years over a pub dinner.
They were born in Ashford, Kent, but were separated when their mother and father split in 1975.
Kemp, who was six at the time, joined her mother in Edinburgh and George, 11, went to east London with his father. They tried to search for one another as adults but had no luck.
Bentley, a builder from East Ham, east London, did not know his sister had married and changed her name.
"Can you imagine the surprise, joy and embarrassment we both felt? It was such a crazy thing," the Daily Express quoted Kemp as saying.
"We had so much in common and really enjoyed each other's company. It was as if we had known each other all our lives," Kemp added.