You have made two great choices for mother figures in this novel. And certainly, Black Mary is a mother figure, too. If you want to use Black Mary, I think it would be fair to say that she operates as Lily's conscience and as her spiritual mother. This is demonstrated by Lily's response to her when she is unable to touch Black Mary's heart because her conscience is not clear and she is spiritually bereft. It is only when the spirit of Black Mary enters her that she is able to touch her heart. Black Mary also acts as a kind of role model for Lily, which is what a mother does. She was able to escape her own chains, what Lily is trying to do.
What I am not clear on is whether or not your choice must be a real character, as opposed to a statue. Perhaps Rosaleen might be a choice to consider for your essay. Rosaleen certainly does mother Lily in the opening chapters of the book. She is the only person who shows Lily any compassion at all, remembering her birthday and offering her consolation when T. Ray punishes her with the grits. She cooks and cleans and does much that mothers do.
You might want to clarify the nature of the assignment with your teacher, to see if a statue is a choice you can make. There is a good case to be made, though, either way.