What is vital to understanding this story is first of all identifying how Dee and Maggie differ, as your question asks, but then you must also go on to consider how Mama's relationship with both of them is different too, which is unavoidable given that the point of view is from Mama's perspective and thus all we know about her two daughters is from her point of view.
Let us start by focussing on Maggie. Consider how Maggie is introduced in the first paragraph:
Maggie will be nervous until after her sister goes: She will stand hopelessly in corners, homely and ashamed of the burn scars down her arms and legs, eyeing her sister with a mixture of envy and awe. She think her sister has held life always in the palm of one hand, that "no" is a word the world never learned to say to her.
This quote clearly establishes some of the central differences between the two sisters. Dee is confident, outgoing, ambitious and determined to make something of life, whereas Maggie is shy, reclusive and passive. Consider how the narrator describes her daughter as a "lame animal" who sidles "up to someone who is ignorant enough to be kind to him". Maggie, described in this fashion, is clearly painted as someone who has such a low sense of self-worth that they are amazed that anyone would actually want to talk to her.
However, the narrator says of Dee, "Hesitation was no part of her nature":
She was determined to stare down any disaster in her efforts... At sixteen she had a style of her own: and knew what style was.
It is clear then that Dee is incredibly self-confident and self-assured. She, as is amply evidenced later in the story, knows what she wants and will not stand for anyone getting in her way, which makes the narrator's decision to not give into her all the more remarkable.
If you want to think about Mama, too, I will add this paragraph. One of the key events in the short story that reveals Mama's character is her refusal to give Dee what she wants, and her insistence that Maggie receives the quilts. It is clear that she loves both of her daughters, but is exasperated by both of them in different ways. However, her decision to give the quilts to Maggie rather than Dee indicates what a high value she places on the family heritage and history, of which the quilts are a symbol. Note too that this is the heritage that Dee has rejected and turned her back on.
I hope this helps you establish the opposite characters between Dee and Maggie in the story - although they are sisters they show themselves to be incredibly different, and Walker could be using them to represent, in Dee, those African Americans that have turned their backs on their family history in their attempt to embrace their African roots, and in Maggie, those African Americans who are perhaps ashamed of themselves and their history.