A Streetcar Named Desire is an elegy to an old South that died in the first part of the 20th century and such elegy is mostly represented in the characters of Blanche and Stella.
The Old south, with its glamour, its historical validity, was a place where kin, family names, traditions, and style was as dominant as it would have been in 18th century Versailles. The cotillions, the gentlemen callers, the Southern Belles, the sumptuous plantations, they all came together as a form of aristocracy.
The early 20th century with its industrial shifts and new ways of dispensing needs without the use of farm hands began to see a delicline in the need of Plantations and southern estates, and the loss of Belle Reve signifies that change in times which left many Southern economies in shambles.
Blanche is a product of both the good and the bad: She was enjoyed the Old South aristocratic living, but she also was a victim of excess and pre-self destructive behaviors probably long before they lost it all. Yet, she kept the qualifiers that would keep her acting and thinking as if she were still back in her hey day- but she is not.
When she comes to see Stella, who is also a star fallen from a formerly glamorous Southern sky, she sees the lowered state of living she is leading with her husband, contrasting it to the live at Belle Reve.
This is why both sisters represent the mourning and elegy to what once was good and plenty and now is dirty and loud and ugly. The question that the story will answer eventually is: Which of the two would pass the survival of the fittest?