Who is Jenna. from Going Bovine and what is their importance?

Jenna is Cameron's twin sister. She is described smart, popular, and able to deal with the realities of life. Jenna loves her brother, but she is tired of being embarrassed by his druggie behavior at school. She is deeply effected by Cameron's illness, and she stays with him at the hospital. She shows up on his quest, and the siblings make peace before Cameron's time runs out.