Jackie Kennedy Was Advised to 'Ignore' Husband Aristotle Onassis' Romance with Maria Callas (Exclusive)

Aristotle Onassis and Maria Callas "couldn't live without the other one,” Kiki Feroudi Moutsatsos recalls

When Aristotle Onassis, the Greek shipping magnate, met Maria Callas, the great opera diva, in 1957 they were both married to others. But that didn’t stop them from falling in love and having an affair that would last until his death in 1975.

“They couldn’t live without the other one,” Kiki Feroudi Moutsatsos, Onassis’ longtime personal secretary, tells PEOPLE.

Once their affair began, she says, they "never stopped seeing each other — never.” Even after he divorced his first wife and later married Jacqueline Kennedy in 1968.

“Onassis was protecting his affair,” recalls Moutsatsos, 75. “He was protecting Jackie, but Jackie was very smart. Everyone in the house was speaking, secretly, of course, that Onassis every night was visiting Maria. She knew. She was discussing it with Onassis' sister, Artemis. She was complaining. Artemis told her not to fight. Just ignore it.”

“Not that it would have been changed,” she adds.

The story of their tempestuous affair is told in the new movie Maria, directed by Pablo Larraín and starring Angelina Jolie as the famous soprano.

Related: Inside Jackie Kennedy and Maria Callas' Love Triangle with Aristotle Onassis from Someone Who Saw It All (Exclusive)

Few were as close to what really happened as Moutsatsos, who began working for Onassis when she was 17 and later wrote the 1998 memoir The Onassis Women that she is looking to republish.

“They often saw each other in Paris,” Moutsatsos recalls. “Their houses were very close.” Onassis had an apartment on Avenue Foch and Callas lived at 36 Avenue George Mandel, both in the posh 16th arrondissement.

As she recounts, “When I was calling every morning, in Athens or in Paris, and I was speaking with the personnel, and they were making jokes, they were telling me Mr. Onassis was very, very tired. And when I was asking why, they were saying again that Maria was visiting.”

“We knew every moment what they were doing,” says Moutsatsos of all the women in Onassis’ life. “Because I was arranging from my office and could make all the details about the trips. Christina [Onassis’ daughter] and Jackie called me to tell me when they would be in Paris and when in the United States.”

But Callas wasn’t always happy with her lover. Says Moutsatsos, “When she was on the boat (Onassis’ yacht, the Christina) she was reading the newspapers and seeing all the rumors — and scandals — about Aristo; they were fighting.”

Related: Angelina Jolie Recalls First Opera Lesson for Maria: ‘I Took a Deep Breath and Just Cried’ (Exclusive)

“He liked to have next to him beautiful women, and Maria was very jealous,” recalls Moutsatsos, who's memoir is the basis for a short film The Heiress, now screening on the festival circuit. "She was very nervous. When articles in the newspaper or magazines appeared, she couldn't stand it. She was always fighting with him.”

Jackie, on the other hand, rarely showed her emotions. Their marriage grew increasingly rocky in later years as they spent time apart. Still Onassis continued to see Callas.

The opera singer never truly recovered after his death on March 15, 1975. Afterward, Moutsatsos recalls, “Maria was desperate. She didn't want to live anymore.”

As for her own turn as the dramatic opera star, Oscar-winner Jolie has said, “I hope what audiences find is that there was so much research done into what we believe she really was like — the human being behind the voice and behind the image. Maybe not ‘behind the voice,’ because the voice is the woman,” she told The Hollywood Reporter.

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“But I think even if I’d read biographies on her, I [wouldn’t have understood her] until I stepped in and kind of felt her. I hope more people discover her and opera and go to the opera and listen to opera.”

Maria is in select theaters now, then streaming on Netflix Dec. 11.