Secret of the Andes, by Ann Nolan Clark

Cusi, a young boy, lives high in the mountains of Peru.  With him live a herd of llamas, his faithful dog, and Chuto, an old Indian man.  It never occurs to him to question the nature of his isolated life until, peering into the valley below, he notices a new family camped there.  There are a mother, father, and two small children.  He sees the love and comfort that exists between them and wonders why he has neither mother, father, nor siblings, and longs to experience that sense of family for himself.

When circumstances show that Cusi is ready, he departs from Chuto with a small herd of llamas to find the ancient city of Cuzco, in hopes of finding a family of his own.  Along the way, though, he discovers that much in the city is foreign to him.  Raised in traditional Incan culture, he learns that most people speak an unfamiliar language (Spanish), have strange churches to a foreign God, and live in a way that he doesn't understand.

Nonetheless, he joins a family with many children (the parents' philosophy is generally "the more, the merrier") as they travel the city to trade before heading home.  But his Indian name doesn't fit (they rename him Nicho), and although they're happy and friendly, Cusi never really feels at home with them.  So he slips out in the night and heads back to the mountain.

Cusi learns over the course of his travels that he is descended from Incan royalty and, like Chuto before him, is charged with supporting the Indians of Peru from the background- remembering and teaching the traditional ways, keeping the language, and making gifts from his herd of llamas to keep the culture alive.  Because although the Incans were conquered, they will be part of Peru forever.