All we know about Sarah (but what more do we need to know?) is that she breaks off her engagement to marry Silas and marries William Dane instead.
Silas's best friend, William Dane, is regarded as "a shining instance of youthful piety" (1.1.7), but he's pulling one over on the whole Lantern-Yard community. His looks give it away: he has a suppressed look of "inward triumph that lurked in [his] narrow slanting eyes and compressed lips" (1.1.7). Come on, Silas! That description is practically code for "liar, thief, and traitor."