Sarah joined the Brotherhood of Steel because her brother, Edward, became a member. When their parents died, Sarah promised to look after her younger brother and take care of him. If this meant following him into the Brotherhood, she was more than willing to do so.
Sarah disliked the military, and just couldn't seem to excel at any of the skills required to get ahead. Everyone knew that she was there because of Edward. And everyone also knew that there was no getting rid of her unless her brother went with her. So, everyone found something for Sarah to do until she took an interest in the medical field. Much to everyone's surprise, including her own, she showed the potential to become a competent doctor. Sarah's place in the world had been discovered.
Sarah is currently an intern at the base. Unfortunately for her, the former staff deserted and she has been left to further her career on her own. She takes her work seriously, and is studying everything she can find related to medicine in the bunker's library.
Sarah Whatley has not appeared nor is she mentioned in any published game. She was to appear only in Van Buren, the canceled Fallout 3 by Black Isle Studios. It is not certain if she will appear in any future Fallout game.