Who is Maggie Sommers from A Test of Wills and what is their importance?

Helena Sommers's sister Maggie is an extremely shy, retiring young woman. She never leaves the little cottage and is extremely intimidated when Rutledge and Davies come out to ask Helena some questions. At the very end of the book, the author reveals that Maggie is clever enough, and aware enough of her mental state, to know that she must die so that her creation, Helena, whom she can no longer control, can die too.