Who is Van's Sister from Dog Sees God: Confessions of a Teenage Blockhead and what is their importance?

Van's sister, Lucy in the comic strip, is in the mental institution after setting fire to the little red-headed girl's hair. When CB visits her and tells her about kissing Beethoven in front of everyone, she is proud of him for daring to be different instead of always being so predictable, and she makes him promise not to apologize when he goes to school the next day.