"I think Warren and Doris do not have it in them to ignore those letters," Riccio said.
Warren Buffett sent his sister $5 million initially to cover the cost of responding to his letters and promised more money if she needed it. The siblings didn't want to say exactly how much Doris has given to the letter writers so far.
Many of the requests are simple: A man who needed a new glass eye. The grandmother who wanted a tombstone for the three children she lost. A disabled woman who needed a car to visit her daughter and grandchildren.
"These are decent people who just didn't have the breaks somebody else did," Doris Buffett said.
She said her drive to help people developed during the Great Depression, when she saw people struggling with such basic needs as hunger and shelter. That desire grew as she faced her own disappointments, including four divorces and the loss of a $12 million fortune in the stock market crash of 1987.
Doris Buffett started the Sunshine Lady Foundation in 1996 after inheriting money. Through it, she has also given away $150 million of her own money, focusing primarily on larger programs such as scholarships for domestic violence victims, college education for prison inmates and efforts to help people with mentally illnesses.
Riccio says Doris Buffett's personal connections to recipients and her willingness to provide so many small gifts sets her apart.
"She cares about people, not about the prestige or the perception of her as a philanthropist," Riccio said.
Doris Buffett focuses her foundation's main giving on the communities where she lives: Fredericksburg, Virginia; Wilmington and Beaufort, North Carolina, and Rockport, Maine. But she doesn't confine her gifts to those places.
Steven Lewicki spent 15 years in prison for a string of bank robberies, but during that time earned his associate's degree thanks to a college program inside Maine State Prison funded by Doris Buffett's foundation.
When he was freed, Lewicki finished his bachelor's degree at the University of Maine at Augusta and got a job with a group that advocates for prisoners.
"I feel an obligation to Doris," Lewicki said. "I feel an obligation to honor her philanthropy and her integrity and her guidance and all of that."
Doris Buffett's main goal is to provide one-time aid and, whenever possible, connect people with other forms of help. But she knows there are limits to what she can do.
"I can't change somebody's life, but I can make it possible for them to do so," she said.