Your instructor has really left an easy out for students with the wording "completely overshadowed." It would be easy for students to qualify "not completely but." However, your paper will be a stronger analysis if you take the stand "Yes she was," or "no absolutely not." My suggestion would be to start out with a three column chart in which you put Rosalind in one column, Celia in another, and a column in between the two labeled similarities. In the characters' columns go through a point by point analysis of character traits, attitudes and behaviors. For example if Rosalind likes apples for breakfast, look at what Celia likes for breakfast. Don't stop there, try to decide why these differences occur or why they're important. In the similarities column explain how the similarities are important to the character development. Good luck.