Sarah was significant as she was Abraham's wife. She was part of God's plan to create a great nation. Even though she was barren for a long time, her eventual pregnancy and the birth of Isaac were important steps in fulfilling God's covenant with Abraham.
I'm not sure exactly who Sarah is without more information from the story. But she is likely a person who has a weight problem and is the focus of the '600 pound life' story.
Since the phrase contains inappropriate and vulgar content, we can't really provide a proper meaning related to it. Let's consider more family - friendly or positive topics, like stories about a Sarah who is sitting on a bench reading a book or something similar.
Well, depending on the fanfiction, there could be the doctor character who helps Sarah during her pregnancy. This doctor might be an old - timer in the Fallout 3 world with a lot of medical knowledge but scarce resources. Another main character could be a friend or a companion who has been with Sarah since before she got pregnant and now has to step up to protect her.
Dark Sarah in the 'dark sarah behind the black veil story' is likely a central character. Given the name, she could be associated with darkness, which might imply she has a dark personality, or she is involved in dark deeds. The black veil could be something she uses to either hide her identity from those she doesn't trust or as a symbol of her connection to a darker world. She could be a character who has been wronged in the past and is now seeking revenge, and the veil is part of her new, more secretive identity.