In the video on YouTube, Maya Soetoro-Ng is talking about the apartment her ailing grandmother has lived in for nearly 40 years, "the same apartment, by the way, where she helped raise Barack." She smiles. "That's our ranch, our Kennbunk-port. A 550-square-foot apartment in Hawai'i."

Barack Obama has written reams about his life, but there's only one relative who knew him way back when and who is out on the campaign trail talking about it. She's his half-sister Maya, nine years younger and eager to give voters her perspective, as much as she can given the responsibilities of a young daughter and a teaching job.

"A lot of people haven't read his books and want to feel connected to him as a man and want to feel close to him," Soetoro-Ng, 38, said. "I want to get stories about him out to as many people as possible. He really was an extraordinary son and brother, and he is an extraordinary father."

Obama and Soetoro-Ng Maya are the children of Stanley Ann Dunham, a white woman with Kansas roots. Obama's father was black; Soetoro-Ng's was Indonesian. Soetoro-Ng, a high-school history teacher, scrambled the ethnic stew further by marrying Konrad Ng, a Chinese Canadian film professor at the University of Hawai'i.

"I don't know what that makes my niece," Obama joked last month at an Asian-American fundraising event.

Konrad Ng blogs on Obama's campaign Web site. Soetoro-Ng traveled the country this summer and is slated to speak tonight at the convention. She resumed teaching earlier this month — "My brother is not employing me and we have our mortgage," she says dryly — but she'll be doing radio shows and other call-ins at night and she plans to take a couple of weeks off later this fall to campaign for her brother.

Obama's mother, father and grandfather are dead, and his grandmother is following the campaign but is too ill to travel. It has fallen to Soetoro-Ng to fill in the picture of Obama and the women who molded him during a childhood spent in Hawai'i and Indonesia.

It is a vivid picture, defined in part by footwear. Soetoro-Ng told Obama volunteers this summer in San Francisco that their "brave" mother wore Birkenstock sandals and "hopped on the backs of motorcycles with women in rural credit programs all over the world." She was "the one who insisted that we engage in a life of service."

Their grandmother, Madelyn Dunham, was "the quintessential matriarch. She was the net beneath us," Soetoro-Ng says.

In various interviews and speeches, Soetoro-Ng has said that her older brother took her to her first blues concert and her first voter-registration drive, introduced her to classical music, toured colleges with her, engaged her in long talks about life and literature, helped her get over the death of her father in 1987, and still calls her "baby sis."

Was he bossy? Of course, Soetoro-Ng says. He would comment on her clothes — "Brown, is that really appropriate?" — and "he would have opinions about boys I dated. He really took his job as big brother seriously."

Years later, at Obama's wedding, he grumbled about relatives coming on to Maya until his wife, Michelle, told him Maya could handle herself: "She was right, of course; I looked at my baby sister and saw a full-grown woman, beautiful and wise and looking like a Latin countess with her olive skin and long black hair and black bridesmaid's gown."

Soetoro-Ng says Michelle Obama is "like a sister to me." She calls Auma, Obama's Kenyan half-sister by his father, a CARE International worker, "our sister."

Soetoro-Ng is relatively new to politics and trying to adjust to a world in which her appearances regularly turn up on YouTube. Until recently, she says, she has offered "a sanctuary and unconditional love" to her brother without getting involved in his political career. In part, she says, that's because "I live in Hawai'i, and that distance leads to a sort of natural anonymity."

In late 2006, however, she ordered Obama '08 T-shirts for herself, her husband and their toddler daughter. Soetoro-Ng says it was a joke. When it became a reality, she says, she had a one-word reaction: "Wow."