Hey, check it out: it's a character without much symbolic resonance. Sarah's a mysterious character, and her motives and character are never fully explained. We know for instance that she's a washerwoman in the neighborhood, but we don't know where she came from. We also don't know, most importantly, why she tried to kill her baby, why she won't see Coalhouse at first, or how she met him in the first place.
What she does is much more important to the plot of this novel than why she does what she does.
What we do know about Sarah is that she's naïve. This is most obvious when she thinks she can go and see the visiting Vice President of the United States about Coalhouse's problem with the volunteer firehouse. This innocence is what leads to her death, and puts the downfall of Coalhouse into motion.