In the Island of the Blue Dolphins sequel how can karana have a niece if her sister and her brother died?

Karana's sister Ulane does not die during the telling of the Island of the Blue Dophin. She applies "make-up" to her face indicating she is unmarried and goes to the mainland with the rest of tribe. Only Karana and Ramo remain on the island and Ramo is killed by the wild dogs. Zia must be the daughter of Ulane.