Isabella is Emma’s older sister who lives in London with her husband and their five children. Isabella is pretty, amiable, and completely devoted to her family. Austen primarily uses Isabella to highlight Emma’s superior intellect. As Mr. Knightley explains in Chapter 4, Emma is, by far, the cleverest member of her family. He elaborates that at ten years old, Emma was able to “answer questions which puzzled her sister at seventeen.” He continues to compare the two sisters and explains that Isabella is “slow” where Emma is “quick” and “diffident” where Emma is “assured.” Isabella, like her father, is also “easily alarmed,” a stark contrast to the confident and usually unaffected Emma. This early juxtaposition of Isabella and Emma’s characters is essential for the reader’s understanding of Emma; it provides a context for her personality. After all, Emma’s inflated sense of her own intelligence and convictions begins to make sense once one realizes that she has spent her entire life surrounded by comparatively slow, excitable people. 

Isabella is also significant because her domesticity provides a contrast to the independent life Emma imagines for herself. In Chapters 11 and 12, Austen zeroes in on the marriage between Isabella and Mr. John Knightley. Isabella is a loving wife and mother, but she is characterized as a simpler, less dynamic woman than her sister, and this characterization implies that motherhood does not require much by way of cleverness or vigor. Isabella’s relationship with her husband is altogether less exciting than the sometimes contentious but ultimately charming back-and-forth we see between Emma and Mr. Knightley, rendering marriage as Emma perceives it to be boring and conventional. Emma maintains that she will never marry because she will never be mistress of as grand of a house as Hartfield. However, by contrasting Emma with Isabella and characterizing Isabella’s marriage as mundane, Austen implies that her independent and opinionated heroine also has no interest in constraining herself within something as lackluster and unfulfilling as a marriage. It is only after Mr. Knightley offers a union based on equality and mutual intelligence in addition to love that Emma reconsiders her previous resolve to never marry.