This woman's parents survived the Holocaust but she only just learned what happened to her sister

Malka Rosenbaum's parents put their life in two mental boxes: one sealing up their memoriesbefore and during the Second World War and another to fill with the life they began afterwards.

Rosenbaum could walk around in the second box, the one theycreated after 1945. It contained her childhood and her family's arrival in Toronto when she was three.

The first box contained a secret: Esther.

A baby born to Rosenbaum's parents while they hid in an earthen basement alongside 10 other Jews during the Nazis' occupation of Poland. A sister she never knew existed until she turned 19. And a part of her history that she never quite understood.

'You had a sister before the war'

"My mother said, 'You have to understand that I worry because you had a sister before the war,'" the Toronto woman, now in her 60s, recalled of the revelation one day in her parents' kitchen. "I mean, I felt like somebody threw cold water at me. I'd never heard of it."

Her parents neverreally talked about Esther again, except to say that they learned she had died while they were in hiding.

But about two years ago, Rosenbaum began to wonder if maybe her parents had been wrong. If, maybe, baby Esther had survived.

Documentary filmmaker Martin Himel asked Rosenbaum's son a similar question and he connected with the descendent of the Polish Holocaust survivorsto spearhead and document her search in his film Secrets of Survival.

The 'before' box

Her search began with her parents, Bina and Haim Postmantier. They were married around the time the Germans invaded Poland in 1939.

The couple became increasingly afraid as the Nazis stepped uptheir persecution of the Jews in their village of Staszow and across the continent. When a young mother named Maria Sczczecinska offered to hide them in herbasement, they accepted, Rosenbaum said.

Had they not, they might have been among the 7,000 Jews forced into the Staszow ghetto created by the Nazis in 1941, Rosenbaum said. All of those people were moved to the Treblinka concentration camp by December 1942.

It's while they were in hiding that Esther was born.

Rosenbaum said her parents had told her they couldn't keep the baby with them; if she cried, it could endanger Sczczecinska, her five children, and the other Jews who werein hiding.

Sczczecinska told her parents about a woman who had just had a child of her own, with whom she could leave the infant, Rosenbaum said of the story her mother had told her. This woman would feed and care forboth childrenduring the war.

After they came out of hiding, the Postmantierswere told that both infants had died of a fever. They stayed in Poland until 1950, when Rosenbaum was born, but never found any evidence their first daughter had survived.

Why her parents kept the secret

But Rosenbaumsaid her aunt wonderedwhether Esther had, in fact, survived. There were stories of Polish families who had taken in Jewish babies during the war and later raised them as their own.

So Himel and a Polishgenealogist set out to find what really happened to the baby girl the Postmantiers were forced to give away.

They find a different story than the one the couple had been told. There's no happy reunion, however, between two long-lost sisters.

Now I feel that my sister is part of my history. She's connected to me.- Malka Rosenbaum

Instead, thanks to the documentary, Rosenbaum was able to meet the descendants of those who helped her parents hide during the Second World War. And she learned that, in fact, her sister had been left on the doorstep of a family living in the forest.

The baby didn't survive, however, although the family looked after her, according to the film's discoveryand Rosenbaumsaid she now has a better understanding of why her parents stayed silent.

"I felt a lot of it was because of their pain, but I think, now,that a lot of it was to shield me from my pain, too," she said."They didn't want me to feel that feeling of loss that they had."

Through the discovery, however,Rosenbaum said she feels that she's gained more than she's lost.

"Before I felt it was a story my parents told me," she said. "And now I feel that my sister is part of my history. She's connected to me."

Rosenbaum'sstory is featured inHimel'sdocumentary Secrets of Survival, which airs tonight at 9 p.m. onCBC'sDocumentary Channel.