The first thing that strikes us about Sophie's mother, Mrs. Wender, who is a more sympathetic character than David’s mother, is that she is very much on her guard and suspicious. She answers the door when David approaches her house with what he as the narrator describes as an "anxious abruptness." He says she looked at him "sharply and frowned" at him.
The next thing is complicated because three traits are bound into one reaction. Firstly, after David's explanation of Sophie's accident, Mrs. Wender's reaction is directed toward Sophie's injury, "Oh!... Her foot!" There be a hidden double meaning yo this since Mrs. Wender knows that it a six-toed foot. Secondly, she suspends her concern for "a moment" to look "hard" at David while she evaluates David's trustworthiness. Thirdly, she makes a decision about the inevitability of trusting David to take her injured child, and says, "Where is she?"
Through the remainder of the two scenes involving Mrs. Wender and Sophie (at the rock and back at the house), she shows tender love for Sophie. This is especially evident when she kisses Sophie's mutated foot that could condemn her to the realm of non-humanity:
[David] watched her mother pause to look down at [her foot] for a still moment, lift it, bend to kiss it gently, and then look up with tears in her eyes.
She also shows tender acceptance of David and courageous trust in him regarding the fearsome they now share, the secret of Sophie’s six mutated toes.
David's mother is a diametrical opposite of Mrs. Wender. David remarks that his mother has a "damping manner of listening only to correct," which strongly suggests she lacks the tenderness Mrs. Wender displays. This is confirmed in her treatment of Aunt Harriet after her abominable suggestion that their babies be switched for the purpose of attaining a certificate that authenticates humanity by confirming normalcy as defined by the Definition of Man.
David compares Harriet to his mother and says he see in her a “little softened version" of his mother. She shows a lack of compassion when she imperviously demands to see Harriet's baby without the wrapping of blankets, calls the baby a “Monster!” and has Joseph order then both out of the house:
My mother said coldly, forcibly: "Harriet, let me see that child--properly. … Tell her to leave the house--and take that with her."
Harriet describes what they see as "a little thing." David suspects it might be a sixth toe like Sophie’s [Though there is no clue, I agree based on one of David's earlier remarks that parallel the “little thing” description: "It seemed a very small toe to cause such a degree of anxiety."] These cold hard traits are mitigated a little when she cries for what seems to be a deeply suppressed secret sorrow after Joseph reminds her "that you yourself have had to make your own penances twice!" This seems to allude to two births of baby's with mutations.