Stepmom (1999)

Three years after divorcing Jackie (Susan Sarandon), the mother of his children, Luke Harrison (Ed Harris) decides to take the next step with his significantly younger girlfriend, fashion photographer Isabel Kelly (Julia Roberts). But, when the flaky Kelly meets Harrison's children for the first time, their fierce allegiance to their mother is obvious. Try as she might, Kelly fails to endear herself to her young charges -- and to Jackie -- until a looming family crisis changes everything.