Who is Olivia Jayne Fuller from Don't Let's Go to the Dogs Tonight: An African Childhood and what is their importance?

Olivia is Bobo's younger sister, the family favorite, a child so lovely that she wins beautiful baby contests. While the Fullers are shopping in town, Olivia wanders into a shallow duck pond and drowns. Bobo feels responsible for the toddler's death and for her mother's subsequent insanity.