"He let me know the world was large, and that I should get to know as much of it as possible," said Soetoro-Ng, 37, who has been campaigning for her brother in advance of Tuesday's Democratic caucuses in Hawaii. "He really was the man in my life even though he was only a teenager when I was a kid. He took on the role of caring for me."
Obama's parents - Barack Obama Sr., a black man from a poor village in Kenya, and Ann Dunham, a white woman whose parents grew up in Kansas - met at the University of Hawaii and married in Honolulu.
After the marriage failed, a 6-year-old Obama left Hawaii to spend four years in Indonesia with his mother and Indonesian stepfather, Lolo Soetoro. It was in Indonesia that his sister was born. In 1971, when he was 10, Obama's mother sent him back to Honolulu to stay with his maternal grandparents.
"Our mom lived in many countries, shared many world views with us and yet the Midwest and Kansas were certainly still very present," Soetoro-Ng said.
Soetoro-Ng teaches history at the private LaPietra Hawaii School for Girls and is married and has a daughter. She said her brother is a private man who deals with questions about his identity and other struggles in "a very personal way."
"He's good though about grappling with them and moving on," she said in a recent phone interview. "Today he is a man very comfortable with himself and peaceful with his sense of self."
Obama honed his ability to appeal to a diverse group of people in the Hawaiian islands, a crossroad of cultures from throughout the Pacific, said Soetoro-Ng.
"Hawaii is the place that gave him the ability to ...
understand people from a wide array of backgrounds," she said.
"People see themselves in him ... because he himself contains multitudes."
His family's own diversity played no small part in developing that skill, she said.
Obama still returns almost every Christmas to visit family, indulge in local sushi, body surf at a beach on the southeastern coast of Oahu and look for sea turtles, Soetoro-Ng said. His parents and grandfather have died, and his grandmother is in poor health but has been following the presidential race closely on television, she said.
"Hawaii really is a sanctuary for him - a safe place where he can just relax, where things are in many respects unchanged," Soetoro-Ng said.
In his 1995 memoir, "Dreams from My Father," Obama wrote about growing up with the island's unique food and culture: poi and roast pig, choice cuts of aku for sashimi and spearfishing off Kailua Bay. Living in his grandparents' downtown apartment, he attended the prestigious Punahou School and drove to parties at Army bases.
Classmates at Punahou describe Obama - known as Barry to them - as an upbeat, social person who played basketball and occasionally wore an African-style shirt.
But in his memoirs, Obama described feeling like a misfit in his Indonesian sandals and old-fashioned clothes when he started at the school. As one of the few black students at Punahou - and among a small group of blacks on the island - he remembered someone wanting to touch his hair and being asked whether his father ate people.
He struggled with his racial identity and turned to marijuana to block the questions out, he wrote.
Former classmate Kelli Furushima, who remembered Obama playfully grabbing a pencil from her ear while passing in the hallway, said she never knew about the turmoil Obama was experiencing. But Furushima said she wasn't surprised.
"You don't let the world know how you feel when you're a teenager," she said. "You might be really insecure inside, but when you're walking down the halls, you're laughing."
