This story in large part deals with how to handle grief - each member of the family, including Susie, has to process his/her own grief and each one does it very differently. Abigail is a prime example. A no-nonsense type woman to begin with, she isn't quite sure what to do with her overwhelming feelings of loss and guilt. Unlike her husband, she is not in touch with her emotions. She reacts in a personally destructive way, seeking physical connection (with Len Fenerman) first, and finally running away.
Much of Abigail's failure stems from her own lack of individuality. Although she gets mad at her own mother for behaving in a "unique" way, it is Grandma Lynn who is better equipped to handle grief. Abigail is just lost, as her own individuality is lost:
When the roll came back from the Kodak plant … I could see the difference immediately. There was only one picture in which my mother was Abigail. It was that first one, the one taken of her unawares, the one captured before the click startled her into the mother of the birthday girl, owner of the happy dog, wife to a loving man, and mother again to another girl and a cherished boy. Homemaker. Gardner. Sunny neighbor.
I've included a link below that deals with the stages of grief and the stages of growth. You might want to review them in connection to your analysis of how Abigail is portrayed in this novel.
I would first start with the superficial characteristics that you are given about the character. What she looks like, what she does, where she lives, etcetera. Then use these to think about what that means about her specifically... How does she react to certain situations? How does she feel when things happen to her? How does her background influence her beliefs and opinions? Use what you are given so that you can really get to know Abigail as a person, then tell what you know.