Human Interest Inside Rosemary Kennedy's Bond with Eunice Shriver, Who 'Had a lot of Pain' Over Sister's Treatment Eunice Shriver turned "her anger into positive action," resulting in the Special Olympics — and the "coolest friendship ever," her family tells PEOPLE in this week's issue By Nick Maslow Nick Maslow Senior Editor, PEOPLE People Editorial Guidelines Published on July 9, 2021 12:04PM EDT Eunice Kennedy Shriver's family is celebrating her Special Olympics legacy for what would have been her 100th birthday on Saturday — and in this week's issue of PEOPLE, the family opens up about Eunice's inspiration for founding her beloved organization 52 years ago: her unbreakable bond with sister Rosemary Kennedy. Born with intellectual challenges in 1918, Rosemary was institutionalized at Saint Coletta community home in Jefferson, Wisconsin, when she was 23. While patriarch Joe Kennedy built the family's political dynasty, nuns cared for Rosemary in secret until 1961, when Joe suffered a stroke and her whereabouts became known. Tragically, a lobotomy in 1941 had left Rosemary in a wheelchair and with greatly limited speech — an injustice that Eunice never forgave. "She had a lot of pain and anger around the way Rosemary was treated," says Christina Schwarzenegger, 29, Eunice's granddaughter. At the same time, Eunice saw possibilities in Rosemary where others saw limits. "She had this extraordinary capacity to turn her anger into positive action," says Tim Shriver Sr., 61, Eunice's son. "She could always draw on the sure conviction that Rosemary was a person who deserved the dignity, joy and opportunity that anyone else deserved." The extended Shriver clan in Hyannis Port, Massachusetts, in 2019. Meet Eunice Kennedy Shriver's Grandkids, Who Are on a Mission to Carry Her Special Olympics Torch Rosemary became a regular visitor at Eunice's Bethesda, Maryland home, where she lived with her husband, Sargent Shriver (who died in 2011), and their five kids. During her weeks-long visits, Rosemary would bond with Eunice during walks, swims and family meals. For more on the Shriver cousins and their work to honor grandmother Eunice Kennedy Shriver, pick up the latest issue of PEOPLE, on newsstands Friday, or subscribe here. "She'd sit with Rosemary, play flash cards with her and try to help with her speech. She'd order all the sweets Rosemary loved — angel food cake, puddings, creams," says Tim, who today serves as the chairman on the Special Olympics board. "It made Mommy happy that she'd found a way to bring Rosemary back to life, back to the family." By giving Rosemary what she missed in the two decades she was hidden from loved ones, Eunice became a "model for a new generation on how to include a sibling with an intellectual disability and not to be embarrassed," says Eunice's daughter Maria Shriver, 65. "That's the big thing. If you see people with special needs going to the White House, which mommy made happen for the first time, then you're like, 'Wow, that can happen.' " Couple Who Wed After Meeting as Special Olympics Coaches Now Have 2 Kids Who Compete in Games Clockwise: Christina Schwarzenegger, Molly Shriver, Tim "Timbo" Shriver Jr., Tommy Shriver, Natasha Hunt Lee, Kathleen Shriver. Courtesy Special Olympics Eunice continued to set an example when her grandchildren visited her homes in Maryland and Hyannis Port, Massachusetts. "I was so amazed by the way my grandmother would hear Rosemary and then translate to all of us," says Kathleen, who remembers how Rosemary would sit with Eunice at the head of the dining table. "And I was like, that is the coolest friendship ever." Known for her infectious competitive spirit, Eunice often found ways for her grandchildren to have fun with Rosemary. "My grandma built this pool with a wheelchair ramp just so that Rosemary could get in, and then she would make me have a swimming competition against 80-year-old Aunt Rosemary," says Molly, laughing. "I don't know many people who know their great aunt. I was fortunate." Kathleen Shriver, Eunice Shriver and Tommy Shriver. Now her grandchildren are working to serve as models for the next generation. In Eunice's memory, six of the Shriver cousins—including Molly, Christina, Tim "Timbo" Shriver Jr., 32, Kathleen Shriver, 27, Tommy Shriver, 21, and Natasha Hunt Lee, 24 — have all formed the Special Olympics Founder's Council. Together they've pledged to volunteer at outreach events, coach and play in unified sports games and raise awareness for their grandmother's cherished cause. "It's really about continuing her tradition," says Christina. That tradition began with Eunice's inaugural Camp Shriver — a summer sports camp for children with disabilities, who at that time were often institutionalized — in the backyard at the family's Maryland home in 1962. Eunice Shriver teaching kids to swim during Camp Shriver's early days in the 1960s. Courtesy Special Olympics, Inc. "Loving someone and seeing the world treat that person callously created for her an energetic determination to prove people wrong," says Tim. RELATED VIDEO: Remembering Jackie Kennedy's Intensely Private White House Trip with Her Kids After JFK's Assassination By 1968 she had launched the Special Olympics, which hosts the World Games every two years. Through grassroots community support and national programs that encourage schools to have unified sports teams that include people with and without disabilities, Special Olympics now helps 5.5 million athletes in 190 countries live out their dreams — on and off the field. "A lot of people think of the Special Olympics as 'nice,' " says Kathleen, whose family will be honoring Eunice at a Catholic mass in Cape Cod on Saturday. "What my grandma was fighting for wasn't a movement about being sweet — it was about changing society." To support the Special Olympics, go to jointherevolution.org, and to learn more about the organization, visit specialolympics.org.