Does a child this age know what it means to be sorry? Is she just saying the "magic" words? Should we continue to make her say she is sorry?
At this age, toddlers usually do not have highly developed social or language skills. Given their lack of verbal ability and interpersonal skills, they often turn to hitting and other forms of aggression (e.g. biting) to express their frustration and anger etc. Removing your child from the scene of the aggression and redirecting her attention to something else is a productive response with this age group. Try being firm and telling her that her hitting hurts and is not allowed. Then leave the room, and leave her to connect your departure with her misbehavior. Over time, this response will result in behavioral changes. Actions such as these have much more of an impact than lecturing, scolding, time-outs and insisting upon an apology. Positive Discipline for Preschoolers, by Nelsen, Erwin and Duffy is an excellent resource for you.