Roberta's mother is a placid foil to Twyla's rambunctious and wild mother:
We were supposed to have lunch in the teachers’ lounge, but Mary didn’t bring anything, so we picked fur and cellophane grass off the mashed jelly beans and ate them. I could have killed her. I sneaked a look at Roberta. Her mother had brought chicken legs and ham sandwiches and oranges and a whole box of chocolate-covered grahams. Roberta drank milk from a thermos while her mother read the Bible to her.
Though neither mother can provide for her child in a meaningful way, Twyla is upset in this moment that Mary is far less dignified than Roberta's mother. Roberta's mother seems to be everything that Mary is not: tactful in religious settings, attentive to her daughter, and modest.
Twyla's foil of the two is ironic for the reader because the two are fiercely similar in the thing which matters most: their inability to perform their responsibilities as mothers. This irony extends to the seeming differences that grow between Roberta and Twyla as the story progresses, including racial and class differences. Though on the surface the two may seem different, no matter how much they diverge, they seem to always come back to each other as former orphans and friends.