Oh, yes, Aunt Sally can DEFINITELY be seen as a mentor to Vyry, who has lost her mother in early childhood. As result, Vyry has no other woman in her life to look up to. In this way, Aunt Sally becomes incredibly important and eventually a source of Vyry's despair.
Vyry bonds to Aunt Sally (the cook on the plantation where Vyry lives as a slave) due to her lack of a mother, but also due to the possibility of Vyry learning from Aunt Sally about how to cook and behave in the big house. It is a boon for any slave to be stationed in the big house instead of in the hot, horrid field. Aunt Sally has this "lofty" station as slave as she cooks for the white family who owns her. After Vyry bonds to Aunt Sally, she is determined to teach Vyry everything she knows so that Vyry can learn how to behave respectfully and to cook well so that Vyry can avoid the fields. The pain between the two when they are parted is very apparent:
Tears were running down [Aunt Sally's] fat black cheeks and she could not control her trembling lips. Vyry stood dazed and numb. Even when Aunt Sally hugged and kissed her, Vyry did not cry. She could not believe this was real, that she would be forced apart from Aunt Sally, that Aunt Sally was leaving and going somewhere.
In conclusion, I have to say that even more appropriate than the term "mentor" would be the term "mother figure." Why? Even though Vyry has aspirations to be as good of a cook in the big house as Aunt Sally, it is the familyesque connection that is more important to Vyry. The separation is so painful that Vyry goes numb.