Ingrid Pedersen says she was given up for adoption as a newborn by Julia Lennon, the singer's mother, in Liverpool in 1945.
She told The Sun newspaper that although she had been aware of her true identity when the Beatles were at their peak, she had never met her older half-brother.
"Now at last I can admit who I am - the little sis John loved but could never find," she said.
Providing copies of her birth and adoption certificates which say she was born Victoria Elizabeth Lennon, Ingrid Pedersen said she had kept her identity a secret for the sake of her adoptive parents, Margaret and Peder Pedersen.
'Time was right'
But Mrs Pedersen died five weeks ago so she had decided the time was right to speak out.
"I kept this secret for the sake of my mum. But now she's dead I want to find my real family," she said.
She added that revealing her true identity would "take a huge weight" of her shoulders.
John Lennon's father Alf walked out on his family three years after his son's birth in 1940.
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Ingrid Pedersen claims that his mother then had a brief affair with a Welsh soldier called Taffy Williams, who was her real father.
The Beatle, who was shot dead in New York in 1980, also had two other half-sisters, born after his mother met John Dykins.
Miss Pedersen says she wants to meet her half-sisters and John's two sons, Sean and Julian.