Who is Catherine from Eat, Pray, Love: One Woman's Search for Everything Across Italy, India, and Indonesia and what is their importance?

Catherine is Elizabeth Gilbert's older sister. She seems to have it all: atheletic fearless, intelligent.... They get along as adults, once Liz at age 28 declares that she is tired of being picked on and Catherine asks what has taken her so long. Catherine does not gloat when Liz's marriage falls apart, even though Liz had always been the favored child.