Both Uses of

Big Brother

in

The House on Mango Street

But after a few months, when the big brother upstairs turned out to be Big Brother, she got on her bicycle and rode through the neighborhood of her high school days until she spotted an apartment with fresh-painted walls and masking tape on the windows.
Chpt Intr *

Big Brother = an organization that attempts to exercise total control over people and that invades their privacy to do so

Uses with a meaning too common or too rare to warrant foucs:

But after a few months, when the big brother upstairs turned out to be Big Brother, she got on her bicycle and rode through the neighborhood of her high school days until she spotted an apartment with fresh-painted walls and masking tape on the windows.
Chpt Intr *

big brother = an older, bigger brother

Definitions:

(1) (Big Brother)
a person or organization that attempts to exercise total control over people and that invades their privacy to do so

This use of the term "Big Brother" was popularized by George Orwell in his novel, Nineteen Eighty-Four. In the novel, citizens were under constant surveillance by an authoritarian government that was personified through a nominal leader, Big Brother.
(2) (meaning too common or rare to warrant focus)
More commonly, especially when not capitalized, big brother generally refers to an older or larger brother. Big Brother can also refer to a member of the organization, Big Brothers.