The mother is hostile toward her daughter Dee and protective of Maggie from the beginning of the story. She holds Dee responsible for the scars of Maggie, both literal and figurative. She resents her daughter's beauty and vigor, saying right before Dee arrives that when Dee was courting "Jimmy T she didn't have much time to pay to us, but turned all her faultfinding power on him." In this statement we see the mother's feeling of rejection and hurt resulting from the way Dee has treated her over the years. So, the mother's refusal to give Dee the quilts is very consistent with her character in that the act constitutes a "payback" for all the hurt Dee has caused. When she feels that Dee looks at her with "hatred" when she says she plans to give the quilts to Maggie, we can imagine the mother drawing a line in the sand, ready to do battle and not give way. Putting all of this in the context of the mother's early statement in anticipating and dreading the arrival of Dee, we see the inevitability of her refusing the quilts. In the second paragraph, the mother says that Maggie thinks that "'no' is a word the world never learned to say to" Dee, and that word, "no," is precisely what she says to her at the end of the story. This word foreshadows the conclusion of the story.
It is a permanent change of character, and I would agree it isn't really a change of character at all. Mrs. Johnson has always been swept away by Dee's vigor and popularity. She takes Maggie for granted, but Dee overwhelms and excites her. Mrs. Johnson has herself always lacked self-esteem; that is why she pictures herself as thinner and prettier. However, she has always valued her heritage and respected her own mother and her own past. That is why she has allowed Maggie to use the priceless family heirlooms. Mrs. Johnson understands that to honor her heritage she must live it, not display it in the way that Dee does.
This is the reason that she refused to give the quilts to Dee. She sees that all of Dee's "fanciness" is really all show. Dee does not understand her own heritage. When Dee fails to listen to her mother's complaints that Dee is a family name and should not be abandoned for something new, readers see the foreshadowing of Mrs. Johnson's final decision.