The Kennedys were the most famous family in England when Joseph became American Ambassador in London. But daughter Rosemary's behaviour began to cause the family increasing concern. Award winning writer Charlotte Jones' play is inspired by the lives of two of JFK's sisters, 'Rosemary' Kennedy, who died in 2005, but underwent a prefrontal lobotomy aged 23 and Kathleen 'Kick' Kennedy who married the heir to the Duke of Devonshire.
The portrayal of Rosemary may be fairly accurate from what I've read. She was able to interact with people but at an 8-9 year old level. I also read that she had trouble understanding her limitations and why so many rules were imposed on her. A sad story. The Kathleen parts were, uh, strange. Maybe her accent was the most authentic thing.