Who is Yvonne Isben from After Ever After and what is their importance?

Yvonne is Tad's younger sister. He refers to her as "ERC" or "emergency replacement child." Yvonne loves her brother even though he is often mean to her. Tad says his actions are meant to keep Yvonne from becoming attached, so she will not be devastated by his death. Yvonne is heartbroken when Tad dies, and Jeffery begins spending some time with her.