A story of family secrets, the love between two sisters and their brother and a drive to help others came to a close on Tuesday when Eunice Kennedy Shriver died at the age of 88.

Shriver was the founder of the Special Olympics for people with learning difficulties, which she built up from a camp for 35 people into an organisation involving the participation of some 3m athletes and more than 150 countries.

Still, she said the heart of her life was not her work but her family. And since she was born into the Kennedy family in the US and married Sargent Shriver, a Democratic vice-presidential nominee, hers was one of the most exceptional and powerful families of the 20th century.

Even today, it is empowered: Arnold Schwarzenegger, the governor of California, is married to Eunice’s daughter Maria Shriver while Teddy Kennedy, beset by brain cancer, still holds a Senate chairmanship.

However, both Eunice and her brother John F. Kennedy were shaped by someone who was not powerful: their sister Rosemary who, as Eunice said, was slower to crawl, walk and speak than her siblings and who never caught up.

Shriver said it was Rosemary’s influence that sensitised John Kennedy to the problems of “vulnerable and weak people”, so underpinning his political philosophy as a whole.

In comments in 2007, she added that the effect on the future president still eludes his biographers. “Not one among the thousands who have written has understood what it is like to be a brother of a person who has mental retardation,” she said.

Soon after moving into the White House, Kennedy set up a research centre to help the mentally handicapped. But it was Eunice who became the champion of people with learning disabilities.

She began by denying that there was any cause for shame in her sister’s condition in a 1962 article for The Saturday Evening Post. “My brother was president at that time [so] I asked him if it was OK,” she remembered years later. “He sent it back after I sent it to him: there were four corrections in it and they were all grammar.”

With its references to the “feeble-minded” the article is sometimes jarring to modern eyes. But in it Shriver excoriated the “medieval prisons” people with learning difficulties were often confined to, and argued that those afflicted had, like everybody else, latent talents, the ability to hold down jobs and the need for recreation.

Years of campaigning and organisation followed – recognised in a hail of tributes this week.

“Most people believe that I spent my whole life really interested in one thing and that one thing is working to make the world a better place for people with intellectual disabilities,” she said in 2007.

“That has been a huge part of our life. But important as it has been it is not the whole story of my life.” Instead, she said she had been lucky – because of the love of her family and the problems she encountered, which helped her develop the confidence she needed to make a difference in the world.

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