#2 identifies the many issues that could be discussed in answer to this question. It is clear that from an external perspective, Dee can be regarded as a hero because she, in spite of her poor roots, has made something of herself in the world and gained social standing. You could argue that Maggie has not been able to do this - she has opted for a very different life from her sister and clearly is very shy and withdrawn compared to Dee. Therefore, you could argue that she is not a hero. However, #2 gives us an interesting interpretation of how she might be regarded a hero. Key to note, however, is that in Mama's eyes, it is Maggie who is the hero for not abandoning her heritage and roots in the way that her sister has.
This is a very interesting question. At first consideration, it might seem that both girls were victimized in various ways by different forces, but upon closer examination, I don't think they were victims because each of them in her own way refused to be victimized.
Growing up an stifling poverty and lack of opportunity, Dee refused to accept what seemed to be her lot in life. She dreamed of a better life beyond the physical, social, and economic confines of her young life, and she refused to give up her dreams. With the help of her mother and her church, Dee does make it into the larger world, to go to college and get the education she so desired. Along the way, she grew to reject her personal family heritage while embracing her greater African-American heritage, which showed a lack of wisdom and understanding, but this does not minimize her efforts to achieve her dream. Dee was selfish in many ways, and this trait might confict with the idea of heroism, but for her to overcome the tremendous odds against her in terms of getting an education and making a new life for herself, her heroic efforts can't be discounted.
Maggie also refused to be victimized, but in very different ways. Like Dee, she was hampered by poverty, but she also had to deal with her childhood role as the second-best sister and with her disfigurement as the result of the house fire. Maggie had many reasons, perhaps, to become angry as the result of her circumstances. However, Maggie chooses not to be a victim. She faces life with heroic courage and humility, choosing love and acceptance over bitterness. Instead of dwelling on her losses, Maggie embraces greatfulness. As a result, her spirit remains intact, and she achieves the final happiness of knowing she will have a marriage and a home of her own.