I got the impression that the refusal was a temporary change. She seems like a loving mother, in general, and I just don't think she could refuse her daughter forgiveness like that. She wouldn't have done anything like it before because Dee had been away from home for a while, and before she left, she wasn't quite so... pretentious. I use that word a lot. I think it's because anything that gets put under the heading "literature" has a tendency toward that, but that is a generalization, and oh well. Anyway, she did it when she did because she wouldn't stand for Dee/W-thing hurting Maggie, who was always well-mannered and never presumptuous like her older sister.
Some details foreshadowing the refusal include all the accounts by the narrator, who is the mother, of Maggie's more sympathy-inducing qualities. She envisions Maggie's awkwardness upon Dee's arrival and mentions Maggie's burns and her feelings of shame about them repeatedly. These accounts show the mother's protective nature, which wouldn't lend itself to allowing anyone to hurt Maggie, even if Maggie said she wouldn't mind.
