Susan Price, Fanny's little sister, acts as a "what-if" scenario for Fanny. The entire time Fanny is visiting her family in Portsmouth, she is continually confronted with questions of "What if?..." What if Fanny has been raised with the Prices? How different would she have been? Or would she still have been similar to the person she became at Mansfield Park?
The sibling relationship between Susan and Fanny directly comments on the theme of family circumstances. This book explores the tensions in the nature vs. nurture debate – do people have a natural, inherent personality or do is their personality a result of their unique surroundings and upbringings?
Susan and Fanny demonstrate how different circumstances can create different people, even within the same family. The same is also true for their mom, Mrs. Price, and her sisters. Susan is in many ways the polar opposite of Fanny, but the two have some important similarities.
Susan and Fanny both have the same instinct to improve things and to help people, and have a similar moral foundation.
Susan was only acting on the same truths, and pursuing the same system, which her own judgment acknowledged, but which her more supine and yielding temper would have shrunk from asserting. Susan tried to be useful, where she could only have gone away and cried; and that Susan was useful she could perceive [...]. (40.4)
Their approaches to doing things (or not doing them) and their overall personalities are quite different, however. Susan is as bold as Fanny is shy, after all. Regardless, the two discover common ground quickly and Fanny begins to slowly grow as an individual while she begins to mentor Susan. At the end of the novel, we learn that Susan eventually takes Fanny's place at Mansfield Park after Fanny marries Edmund.