These two paragraphs are from Toni Morrison's commencement address to the Barnard Class of 1979 (link), where she talked about Cinderella:
What is unsettling about it is that the story is essentially the story of a household, a world, if you please, of women. Of women gathered to abuse another woman. There is, of course, a vague rather absent father and a nick-of-time prince with a foot fetish. But neither has much personality. The real fireworks don’t concern the men, and do not take place among or between them. The surrogate “mothers” (god- and step-) contribute to Cinderella’s grief and to her release and happiness.
But it is her stepsisters who interest me. How crippling it must have been for those young girls to grow up with a mother, to watch and imitate that mother in the enslaving of another girl. How brutalized the sensibilities must be when you are encouraged, instructed, expected to live off the selfless labor of another woman. How poisonous to be forever in the company of a non-nurturing mother — a mother without milk.
In the second paragraph, if we were to replace must have been with must be, would the sentence become ungrammatical?