This is a period piece. Not sure how there's people surprised? If she did anything to the sister she would've been hung. And considering her upbringing she wouldn't know how to get back at her sister without outright harm. The guy had to step in or things would've been much much worse. And considering absolutely no one else is willing to help her, or give a crap about her, it absolutely had to be him...
Or she could've just stopped herself but she's clearly too far gone for that.