@Yansas
I probably should have clarified that I meant from an outside point of view. MC is doing everything out of self interest, but from other people's perspective she's an older sister helping her younger sister.
As for the second part of your comment, there really are people who are that naive. I worked with a guy who honestly thought him being treated worse than everyone else was or bosses trying to motivate him despite everything everyone else tried to tell him. My sister is dated a guy she refused to believe was a scumbag because he's always nice to her. Or it could just be poor writing on the author's part trying to make an innocent character and not fully plotting out how she fit into the story.
Not trying to change anyone's mind, just throwing out my thoughts and interpretations since I find stories more enjoyable when using the information provided to add more depth to characters than bad guy = bad and good guy = good. Nothing wrong with dumping all your hate onto a character though.