Unfortunately, when I read this story I don't see any proof that Mama does actually love both of her daughters. Her affection for Maggie is of course clear through the way that she stands up for her and protects her, however in everything she says about her relationship with Dee, it seems clear to me that she does not love her in the same way as she loves Maggie. Consider, for example, the way that she talks about her new house and how Dee responds to it:
This house is in a pasture, too, like the other one. No doubt when Dee sees it she will want to tear it down. She wrote me once that no matter where we "choose" to live, she will manage to come see us. But she will never bring her friends. Maggie and I thought about this and Maggie asked me, "Mama, when did Dee ever have any friends?"
Dee is presented by Mama as a character who is ashamed of her origins and does everything she can to escape them. Her letter saying she would visit her mother in sister no matter where they "choose" to live reveals her snobbery and her inability to comprehend that many people don't have any "choice" at all in where they live. Her choice of language speaks of condescension and a patronising attitude that Mama is very aware of. Therefore it seems clear that Mama does not have the same kind of feelings for both of her children.