Glimpses into the family life of the famously reclusive author of To Kill A Mockingbird, Harper Lee, have been given by her sister Alice, a practising lawyer who recently turned 100.

Alice Finch Lee, known as Miss Alice, was speaking to documentary maker Mary McDonagh Murphy. Although her sister Nelle Harper Lee gave her last interview in 1964, Miss Alice was persuaded to talk to Murphy, telling the filmmaker that her sister "grew up quite the little tomboy", and that she later became an author who "did not think that a writer needed to be recognised in person and it bothered her when she became too familiar."

Murphy saw Miss Alice at her 100th birthday party, thrown in the Monroeville, Alabama, offices of her law firm Barnett, Bugg, Lee & Carter (but not attended by her sister Harper, who went instead to a family-only gathering two days later). Asked by Murphy how she had lived so long, Miss Alice said: "I don't do anything to bring on dying. I live day by day." Her nephew Hank Conner told the documentary maker, whose film Hey, Boo: Harper Lee and To Kill a Mockingbird was released to mark 50 years since the novel was published in 2010, that "Southerners are always attributing things – good and bad – to genes and breeding. Miss Alice comes from good stock."

Conner also told Murphy about how his other aunt, Nelle Harper, would play the original cast album from Annie Get Your Gun, and how, later, "manuscripts" would arrive at the sisters' childhood home on South Alabama Avenue addressed to Miss Alice from New York. At the time, Harper Lee was living in the city, working by day as an airline ticket reservationist, and writing by night. "Alice is a very good editor and a very good copy editor," he said.

News of the manuscripts, plural, will be greeted with interest by the author's millions of fans: Lee has not published another novel since To Kill A Mockingbird took the world by storm in 1960, although she said in 1964 that she was working on a second novel "and it goes slowly, ever so slowly". She dedicated her debut, a Pulitzer prize-winning story of racism in the American south, to her sister Alice and her father, "in consideration of Love & Affection".

Miss Alice revealed to Murphy how she was asked by her father – the inspiration for Atticus Finch in To Kill a Mockingbird – if she would like to join his law firm after she passed her bar exam in 1943. "He wasn't pushing or anything," she said, "He was just being like he'd always been: 'Do your own thing but do it well.'" She had two questions for him. "When you grow up in one town, you are always Mr Lee's little girl. 'Would I be an adult separate and apart from you?' Daddy said, 'I think you've been gone long enough for that not to happen.'" Then she asked: "'How is a small town going to react to a woman in a law office?' There were not many around those days. And my father smiled and said, 'You'll never know until you try.'"

She has worked at Barnett, Bugg, Lee & Carter ever since. She does the New York Times Sunday crossword every week, still reads voraciously, enjoying "splashy criminal proceedings", and sends herself to sleep by naming presidents. "She recites the names of all the presidents of the United States," wrote Murphy in a piece for the Daily Beast about her interview with the 100-year-old lawyer. "If not asleep by the time she gets to Obama, she moves on to list all 53 governors or all 67 counties of Alabama. If still awake, she goes to work on all the vice presidents of the United States or all the first ladies – of Alabama."