Name one or more modern additions Sexton makes to the familiar tale of Cinderella. What point does she make about love, marriage, or another aspect of the fairy tale by making that change?

Anne Sexton's poem "Cinderella" is a cynical modern-day interpretation of the famous fairy tale. She is especially acerbic in her take on Cinderella's relationship to Prince Charming. In the original story he sweeps Cinderella off her feet and in true fairy-tale fashion, they live happily ever after. They do in Sexton's poem too, but not quite in the same way:

Cinderella and the prince lived, they say, happily ever after, like two dolls in a museum case never bothered by diapers or dust, never arguing over the timing of an egg, never telling the same story twice, never getting a middle-aged spread, their darling smiles pasted on for eternity. Regular Bobbsey Twins. That story.

Sexton resents the happy couple for not being subject to the normal, everyday problems of a modern suburban husband and wife. The character of the wicked stepmother also gives Sexton an opportunity to cast a jaundiced eye over another aspect of modern family life, namely stepfamilies:

No, Cinderella, said the stepmother, you have no clothes and cannot dance. That's the way with stepmothers.

The suggestion here is that there's nothing out of the ordinary in how the wicked stepmother treats Cinderella; it's just how stepmothers are. Ugly stepsisters are not much better. Not only are they cruel, they're also prepared to commit acts of self-mutilation in order to make their feet fit the glass slipper. The eldest slices off her big toe and then puts it on top of the slipper, vainly hoping that somehow Prince Charming won't notice and will choose her to be his bride. The other sister has no better luck; she cuts off her heel and, like the eldest, bleeds profusely.
The ugly sisters allow Sexton to make a wry comment on contemporary gender relations. In particular, they're used to highlight the undignified lengths to which women are often expected to go in order to obtain a husband.
https://www.units.miamioh.edu/technologyandhumanities/sexton.htm