Stowe provides further details about her story based on actual events in A Key to Uncle Tom’s Cabin.
Stowe believed that everyone knew a Marie St. Clare. Whether you lived in Europe, the North or the South, you knew someone like Marie. The difference between Maries or the North and Maries of the South was that in the South they were just like the character in Stowe story and in the North she treated her servants poorly, paying them a low wage and expecting them to willingly do the worst work just to please her.
The full story of this connection can be read in Chapter VIII , “Marie St. Clare” of A Key to Uncle Tom’s Cabin.