You make a very astute observation. It is clear that the fire is of fundamental importance in terms of the relationships between Mama, Dee and Maggie, and that although it happened over a decade ago, the way that it still impacts these three characters is very clear. Note what we are told about the fire and how it reveals character:
Sometimes I can still hear the flames and feel Maggie's arms sticking 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. 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 grey board of the house fall in toward the red-hot brick chimney. Why don't you do a dance around the ashes? I'd wanted to ask her. She had hated the house that much.
Note the way in which Maggie is wounded and injured, an event that scars her up until the present and also makes her incredibly dependent upon Mama. Also, the tension between Dee and Mama is perfectly evident, as Dee's feelings towards their old home and its humble nature and her delight to see it burn down make clear. Although this conflagaration occurred over a decade ago, we can therefore see that the relationships of these three women are still marked by it, indicating how the fire is still smouldering in their lives.