American playwrights have often used siblings within a family to stand for divisions within the self or for two opposing forces. Consider the relationships between Blanche and Stella in A Streetcar Named Desire and Biff and Happy in Death of a Salesman.

Blanche and Stella initially appear to be opposites. Blanche is attached to her high-society past and the Old South as represented by the lost Belle Reve. Stella left Belle Reve long ago to live with the decidedly less-than-romantic Stanley in his less-than-romantic hovel of a home. As Stanley says, she loved being taken off the "columns" of her formerly pampered, privileged life. From these initial impressions, it would seem that Stella is more realistic.
However, the two sisters are both in their own ways self-deluding. Blanche lies to herself and others, to the point where even she loses track of what is real and what is not. In the end, Stanley's raping her causes a complete psychic break—a representation of ugly reality overwhelming fantasy.
Stella is little better, though: she makes excuses for Stanley even after he beats her, and at the end, she seems to struggle to not believe Blanche when she tells her about the rape. Eunice goes as far as to tell Stella that she needs to keep on believing Stanley is not a brute just so her life won't be more uncomfortable. So in the end, both sisters are more similar than different.
Biff and Happy are the inverse—they seem very similar, both being selfish and lying to their father. However, in the end, the two are quite different. Happy remains committed to his father's view of the American Dream, obsessed with success and material wealth. Biff, even though he seemed like a shoe-in for the traditional American Dream with his football glories in high school, looks at himself honestly and rejects his father's idea of what makes someone a success in life. He has an epiphany, while Happy remains trapped in a delusion.