So, if you know that then why keep responding to your sister like you are the hero and she is so less than?

Anyway, your sister isn't a sociopath, so she would eventually respond to unconditional love, if you could gain her trust.  That is what we are talking about here.

What never changes is the core reasons why people behave in a certain ways, so I can say with certainty that your sister makes life plans with abusive people because she believes that she deserves no better.  

Where did she come up with the belief that she is *so* undeserving? The answer is always that it was implanted there when she was a child, mostly by adults.  

Child psychology isn't rocket science, and people can change with the right input to change their preconceptions, but if you keep clinging to black and white beliefs like: you're the successful child and she's the failure then, of course, nothing will change.  

Of course, changing how you see her also means changing how you see yourself.  The question is: are you willing to do that to help your sister and her kids, or do you enjoy your hero status way too much?