At the beginning of the novel, Aunt Bobbie barely figures into the story. She may live in the same house as Nikki and the kids (though their sections of the house are separated), but she keeps to herself and minds her own business… even when she knows that there is some child abuse and neglect going on. Because of this, Matthew pretty much gives up on her and thinks that she'll be no help.
But Aunt Bobbie surprises them. Once Murdoch makes it clear to her that it's her responsibility to help out with the kids, Aunt Bobbie rises to the task. She works with Murdoch and Ben tirelessly to get custody of the children, and happily fills the role of surrogate mother:
Sometimes I look at this new Aunt Bobbie, the one who plans college applications with me, who cheers herself hoarse at Callie's field hockey games, and who reviews your homework and braids your hair. She looks the same as ever—a plainer, inflated version of Nikki. But that other Aunt Bobbie heard just about everything her sister said and did, ten feet above her head, and she never said a word to interfere or even to offer us comfort. Then she became this new person. Just like that. (22.4)
Aunt Bobbie becomes a protector and a guardian to the children when they least expect it. She stands up to her good-for-nothing sister and takes care of the kids so they don't constantly have to look out for their own safety. They can just grow up and know that they'll have a normal adult figure that will look out for them. Finally.