The sister isn't an awful person, but she's neglecting her responsibilities and turning a blind eye to Athanasia's misfortunes. It's not hard to see, or it shouldn't be if you've lived with a person for all of their life, what they do because they want to and what they do because they should.
It doesn't matter what her sister has to lose, if you're going to say something like 'true love' and play the role of the good girl, while not standing up to your own sister, then are you really 'good'? If you're going to play it right, do it until the end. In reality, this 'evil lady' is the hidden 'good girl' isn't she? Considering that she's taking responsibility for her sister's mistakes without complaint, doing so like she's used to it. Being 'used to' being an outcast, 'used to' being a scapegoat and misunderstood, isn't that a bit sad?
Thing is, the sister 'tried' to stand up for her sister, but how hard did she try? She threatened to kill herself in order to marry somebody she loves instead, yet she can't get seriously upset that her sister is being talked rudely about? I don't know about that.