British royal family patriarch Prince Philip, 96, has lived a long and full life. He married Queen Elizabeth in 1947 and has since welcomed four children, eight grandchildren, and five great-grandchildren. But Philip’s older sister, Princess Cecilie of Greece and Denmark’s life was tragically cut short on a foggy day in Belgium in 1937.

Philip, Cecilie, and their three siblings are the children of Prince Andrew of Greece and Denmark (son of King George I of Greece) and Prince Alice of Battenberg (great-granddaughter of Queen Victoria of England). The family was exiled from Greece in 1922 and settled in Paris. And in 1931, Cecilie married Georg Donatus, Hereditary Grand Duke of Hesse — or “Don” for short — with whom she would raise three children.

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Actors portraying a young Philip and Cecilie on _The Crown_.

Then, tragedy struck. On Nov. 16, 1937, Don and Cecilie departed Frankfurt, Germany, en route to London for the wedding of Don’s younger brother Louis. They took off on a flight operated by Belgian airline Sabena, along with two of their three children, Don’s widowed mother, Louis’s best man, the children’s nurse, and four crew members.

The flight was scheduled to stop in Brussels, Belgium, but bad weather forced the pilot to continue on to the Belgian city of Ostend. The weather there wasn’t much better, however, and in the heavy fog, the plane clipped a factory chimney. A wing and an engine tore away from the fuselage, and the plane crashed to the ground, killing all 11 passengers onboard, according to Allgemeine Zeitung. Don was 31 at the time, and Cecilie was just 26.

Even worse, Cecilie was eight months pregnant, and a Belgian inquiry into the crash revealed the pilot was attempting to land in Ostend because she was giving birth. (The baby’s body was later found in the wreckage.) Cecilie and Don’s other child, Princess Johanna, died of meningitis in 1939 at the age of two.

The story has gets even darker. Cecilie and Don were both members of the Nazi Party, and photos from their funeral show Prince Philip surrounded by people performing the Nazi salute, The Independent has reported. The future consort, Philip, was only 16 at the time, and we have to imagine it’s a chapter of his life he’d prefer to forget for a number of reasons.

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