Barbara Sinatra, Frank Sinatra's fourth and final wife, died Tuesday, nearly 20 years after her husband's death. She was 90 years old.

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According to a representative for the Barbara Sinatra Children’s Center Foundation, she died at her home in California after her health had been on the decline for month.

"She died comfortably surrounded by family and friends at her home," Children's Center Director John Thoresen said.

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Barbara Sinatra married Frank Sinatra in 1976 and went on to stay married until his death 22 years later, making it the singer's longest marriage. They do not share any children.

During her lifetime, Barbara Sinatra was an advocate for victims of child abuse and began her foundation in an effort to provide counseling and other services to victims.

Barbara Sinatra, wife of late entertainer Frank Sinatra, at the National Italian American Foundation's 33rd Anniversary Awards Gala on October 18, 2008 in Washington, DC. The event, which takes place in Washington each year, celebrates the achievements of Italian Americans.  (Photo by Brendan Hoffman/Getty Images)

Credit: Brendan Hoffman

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Credit: Brendan Hoffman

“Barbara started raising funds for it in 1985 with Frank’s support,” said Thoresen. “It opened in 1986 and since that time, over 20,000 children have received beneficial therapy here. There are several child advocacy centers like it around the country, (but) this one is probably most recognized.”