There’s a photograph that Ahmed Rahim snapped of his sister Reem in the Grand Canyon in the summer of 1998, seated on a ledge over an immense drop. The image — Reem posing next to the abyss — commemorates a moment: The pair had just discovered they shared the same idea — to import numi, a dried lime brewed as tea in their native Iraq, and start a business together. Their resulting company, Numi Organic Tea, is now one of the largest importers of Fair Trade-certified tea in the world, with annual sales north of $20 million in nearly 40 countries. Ahmed, 45, is CEO and “chief alchemist.” Reem, 47, is creative director, focusing on design, copyright, and new product packaging.
After growing up in Cleveland, where their family immigrated in the early 1970s, Reem and Ahmed (the youngest of four siblings) followed artistic paths in their twenties. Reem studied drawing and painting in Italy and earned a master of fine arts from John F. Kennedy University in Pleasant Hill, Calif. Ahmed worked as a photographer and filmmaker throughout Europe and opened several teahouses in Prague.
When they launched Numi, they worked out of their 750-square-foot apartment in Oakland to design the first packaging — a painting by Reem based on a photograph Ahmed took of a shadowy passageway in Morocco. The company moved three times between 1999 and 2004, when Numi found its home in the 25,000-square-foot Oakland warehouse it currently occupies. The fast growth affected Reem and Ahmed’s relationship. “There were times probably when we wanted to walk away from the business because we wanted to walk away from each other,” says Ahmed, who shouldered responsibility for finance, operations, and sales.
MORE: Face to Face – Steve Case and Ted Leonsis
Instead, they decided to set their egos aside and underwent therapy together. Reem, shy by nature, says Ahmed doesn’t avoid conflict and taught her to speak up more. “Every time I’d get irritated, he’d say, ‘Just say it,’ ” she recalls. From his sister, Ahmed learned “to appreciate more, to celebrate more, to really hear people and let them know you’re hearing them.”
Today the siblings are working on the Numi Foundation, which helps foster education, art, and health initiatives throughout Oakland. Soon Ahmed hopes to expand their efforts to their birthplace, Baghdad.
Reem Rahim on business with her brother: “The deepest thing is that you have love for one another and want to work things out.”
Ahmed Rahim on business with his sister: “You’ve got to really give in more and listen and support the other.”
This story is from the November 18, 2013 issue of Fortune.