Why does Maggie have a real smile at the end of the story?

Maggie has a real smile at the end of this story because Mama has finally told Dee, Maggie's sister, no.  Dee has come from out of town, sweeps into Mama and Maggie's house, demanding items that were hand-made or owned by various family members, items that her mother and sister still use on a daily basis.  Dee's demands are accommodated, the same way Dee has always been accommodated.  The last object she requests is a pair of old quilts, hand-sewn by family, but Mama tells her that those quilts are promised to Maggie.  Dee becomes immediately possessive, even backing away from Mama when she tries to touch the quilts, and she says that Maggie would be "'backward enough to put them to everyday use'" (Dee would hang them, not use them).  When Maggie tells her mother that Dee can have them, something comes over Mama.  She snatches the quilts out of Dee's hands and drops them into Maggie's lap, telling Dee to choose from among the other quilts.  Dee is shocked at having been denied something she wants, and she storms away, leaving Mama and Maggie in peace to enjoy their home and each other's company.  Maggie can't help but smile because Dee has been put in her place for the first time, and because it seems that Mama has found a new appreciation for and understanding of Maggie.  Mama put Maggie's wants first.