The Wicked Lady.

Caroline is to be wed to Sir Ralph and invites her sister Barbara to be her bridesmaid. Barbara seduces Ralph, however, and she becomes the new Lady, but despite her new wealthy situation, she gets bored and turns to highway robbery for thrills. While on the road she meets a famous highwayman, and they continue as a team, but some people begin suspecting her identity, and she risks death if she continues her nefarious activities.