Who is Emma from Wintergirls and what is their importance?

Emma is Lia's eight-year-old stepsister, and the one person in the world that Lia truly cares about. Emma is a vivacious, energetic, and love-filled child. She adores Lia, but has told her friends that Lia is suffering from cancer, which is why she's so thin and why she's been in and out of the hospital. Emma is one of the main reasons that Lia wants to get better after she almost dies. Her family says that she won't be able to see Emma until she's well again, and this is enough incentive for Lia to choose to get better.