The only thing that saves Stepmom from being a total failure is the acting of the three leading ladies: Susan Sarandon as Jackie (the mother), Julia Roberts as Isabel (the step mother), and Jena Malone as Anna, one of the children who is caught between them.
Luke (Ed Harris) falls out of love with Jackie and falls in love with Isabel. Luke and Jackie's two children don't think much of their new soon-to-be step mother and proceed to make life miserable for her, encouraged in part by Jackie. As time passes by, we see Jackie coming around to accepting Isabel, especially as she finds out she is diagnosed with cancer.
The movie drags a lot, as the tear-jerking scenes are milked for all their worth. Half-hour into the movie, the outcome is completely predictable. I recommend skipping this one unless you enjoy being emotionally manipulated by Hollywood in this particular manner.