A key element to this story is the difference between Maggie and Dee. By recalling the house's burning, their mother is able to emphasize how different the girls are from one another.
During the fire, Maggie was burned very badly (scarred as a result), so much so that her mother
...can still hear the flames and feel Maggie's arms stickign to me, her hair smoking and her dress falling off her in little black papery flakes. Her eyes seemed stretched open, blazed open by the flames reflected in them.
Dee, however, was unhurt. In relation to Dee's character, it is extremely telling that, during a time when her sister and mother are suffering physically and emotionally, Dee simply stands and watches.
And Dee. I see her standing off under the sweet gum tree she used to dig gum out of; a look of concentration on her face as she watched the last dingy gray board of the house fall in toward the red-hot brick chimney.
At a time when Dee should have been mourning the loss of her family's home and trying to help her mother and sister in any way possible, not to mention being mortified by her sister's injuries, she is inwardly celebrating the collapse of a building she considered to be less than what she deserved.
Understanding Dee's behavior during the fire allows the reader to understand her lack of appreciation for her culture and family heirlooms. She is concerned only with herself and her own desires. Maggie, however, appreciates what she has. She belongs to that culture and represents all that is good about it. Maggie has never turned her back on who she is, and is not concerned about appearances; her scars have only served to reinforce her belief in what really matters, which is not appearance.