No. She's of the lower class and would have a greater respect for the consequences, possibly having had brushes with the law before. Those of a higher class tend to believe themselves above consequences, sometimes accurately, and therefore it's more likely that a sudden passion would have overcome the better sense of the sister than the governess.

[He shrugs, hands spreading.]

Though the governess was still a much more intelligent guess than that of the three poor fools the police interrogated.