Not sure if this is the same book but sounds similar to "Mutant Message from Forever" by Marlo Morgan.
Synopsis taken from Amazon site: "the story of aboriginal twins who grow up apart. A boy is adopted by an American family and raised in the US with little sense of who he really is or of his cultural heritage. His sister is sent to an orphanage in Australia where she learns nothing of abriginal culture and encounters continual racism. After a series of shattering losses, she strikes out into the desert, the land of her ancestors, where she discovers the spiritual truths that have been missing in her life. Becoming a 'runner between the worlds', she begins working as a political activist. Her work brings her into contact with her brother, but at first neither is aware of their true relationship to each other. The twins grow and change as they share their experiences with one another.