Kara Kennedy, the oldest child of the late Sen. Edward Kennedy, died at a Washington-area health club, her brother said Saturday. She was 51.

Patrick Kennedy, a former congressman from Rhode Island, said his sister died Friday.

"She's with dad," Patrick Kennedy said. Their father died in 2009 at age 77 after battling a brain tumor.

Kara Kennedy had herself battled lung cancer: In 2003, doctors removed a malignant tumor. Patrick Kennedy said that his sister loved to exercise, but that he thinks her cancer treatment "took quite a toll on her and weakened her physically."

"Her heart gave out," he said.

Kara Kennedy was the oldest of three children. She and her brother Edward Kennedy Jr. helped run their father's U.S. Senate campaign in 1988. The National Organization on Fetal Alcohol Syndrome lists her as a national advisory board member on its website.

Edward Kennedy Jr. himself was a survivor after losing a leg to bone cancer as a child. And Patrick Kennedy had surgery in 1988 to remove a non-cancerous tumor that was pressing against his spine.

In 1990, Kara Kennedy married Michael Allen. The couple had a son and daughter, both now in their teens.

Kara Kennedy was born in 1960 as her father campaigned for his brother, John F. Kennedy, during the presidential primaries.

The late senator wrote in his 2009 memoir, "True Compass," that "I had never seen a more beautiful baby, nor been happier in my life."

She was diagnosed with lung cancer in 2002 and was given a grim prognosis by doctors, her father wrote.

In the book, Edward Kennedy recalled her operation, along with her aggressive chemotherapy and radiation treatments.

"Kara responded to my exhortations to have faith in herself," he wrote. "Today, nearly seven years later as I write this, Kara is a healthy, vibrant, active mother of two who is flourishing."

In 2009, shortly before his death, Edward Kennedy was awarded the Presidential Medal of Freedom by Barack Obama. Kara Kennedy accepted the award on behalf of her father.

In an April article she wrote for The Boston Globe Magazine, Kara Kennedy recalled the lessons her father taught his children.

She wrote of family trips in the summer when the late senator would lead his children on explorations of historic battlefields and buildings, trips she said taught her that one person can make a difference.

"What mattered to my father was not the scale of an accomplishment, but that we did our share to make the world better," she wrote. "That we learned we were part of something larger than ourselves."

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Associated Press Writers Jessica Gresko in Washington and Jay Lindsay in Boston contributed to this report.