The presence of William reinforces the suitability of Edmund as a partner for Fanny. While Henry represents a base, physical kind of love, William represents the ultimate in spiritual, Platonic love. Edmund is a happy medium between the two, brotherly yet still available as a sexual partner for Fanny. Mary seeks to draw him over into the base side of love, and she nearly succeeds. Only her greed in wishing to secure a better income for him before she commits destroys her plan. Her brother's motives are closely aligned with hers, although it is not money he is after. His pursuit of Fanny will lead the plot into some morally ambiguous territory; the very same qualities that make wooing Fanny a challenge are the qualities that make her a worthy partner, and to many, Sir Thomas and Edmund included, Henry seems to be displaying his good taste and moral excellence, not his love of sport, in courting Fanny. She also promises to be a good influence on him; perhaps their marriage would be beneficial overall for its effects on his character, despite the damage to Fanny's happiness.
William, Edmund, and Henry are compared in another way, too: their choice of occupations. William's career at sea has made him a glamorous figure; he is brave, committed, and hard-working, but he has not yet been rewarded with a promotion. Edmund is also committed and will probably be hard-working as a parson; he, however, has the luxury of a wealthy family to ensure that he does not need to worry about money. His decision to take orders is as much a philosophical position as it is a career move; he doesn't need to live at Thornton Lacey and give the sermons himself, but he will. This highlights his essential incompatibility with Mary and angers her most. Henry, of course, has no career; he is a wealthy young heir. His daily "work" involves entertaining himself, and does not contribute to character formation in the way that Edmund's and William's livelihoods do/will. The choice of a career for the male characters in this novel parallels the choice of a husband for the female characters, as financial pressures often come into conflict with more noble considerations, and the proper choice is not always simple or obvious.