LOS ANGELES - Barbara Sinatra, the fourth wife of legendary singer Frank Sinatra and a prominent children's advocate and philanthropist who raised millions of dollars to help abused youngsters, died Tuesday. She was 90.

Sinatra died of natural causes at her Rancho Mirage, California, home surrounded by family and friends, said John Thoresen, director of the Barbara Sinatra Children's Center.

With her husband's help, she founded a nonprofit center in Rancho Mirage in 1986 to provide therapy and other support to young victims of physical, sexual and emotional abuse.

In the years since, Thoresen said, more than 20,000 children have been treated at the center and hundreds of thousands more worldwide through videos it provides.

A former model and Las Vegas showgirl, Barbara Sinatra was a prominent Palm Springs socialite in her own right before she married her husband in 1976, when he was 60 and she 49. They remained wed until his death at 82 in 1998.

It was her third marriage, Sinatra's fourth and the most enduring union for both.

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She met Sinatra through her second husband, Zeppo Marx of the famous Marx Brothers comedy team. The couple had been close friends and neighbors with Sinatra in Rancho Mirage until she left Marx for the singer in 1973.

Frank Sinatra, then single, had previously been married to his teenage sweetheart, Nancy Sinatra, the mother of their children, Nancy, Tina and the late Frank Jr.); Ava Gardner, who died in 1990; and Mia Farrow.

A notorious womanizer throughout much of his life, Frank Sinatra didn't ask his fourth wife to marry him until she threatened to leave if he didn't, she recalled in her 2011 memoir, "Lady Blue Eyes: My Life With Frank Sinatra."

Both played prominent roles at the children's center after she founded it in 1986.

"Frank would come over and sit and read to the kids," Thoresen said of the sometimes volatile entertainer.

"But the best way she used Frank," he added with a chuckle, "was she would say, 'I need a half-million dollars for this, so you do a concert and I get half the money.'"

She remained active at the center until recently, pushing for creation of the video program just last year, raising funds and dropping by often to make sure the children had what they needed, Thoresen said.

Already a socialite in the Palm Springs area through her marriage to Marx, Sinatra mingled with such celebrities as Dinah Shore, Marilyn Monroe and Tony Curtis, raising money for numerous charitable causes before establishing the children's center.

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Those years were a far cry from earlier, more modest ones she described in her memoir.

Born Barbara Blakely in Bosworth, Missouri, she recalled growing up poor and friendless.

She moved with her family to Wichita, Kansas, when she was 10 and to Long Beach, California, at 18. It was in Wichita where, like millions of other teenage girls in the 1940s, she fell in love with a voice on the radio.

"Frank Sinatra had always been part of my life — from when I first heard the singer everybody was talking about as a 15-year-old butcher's daughter," she said.

Her first marriage, to a Sinatra-style singer, was brief as he struggled to find work. They split shortly after the birth of her son and she married Marx in 1959.

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She is survived by her son, Robert Oliver Marx, and a grand-daughter, Carina Blakeley Marx.

Funeral arrangements are pending.