Her sister is the actual inheritor of the family, so if she's gone she'll get everything from her mother....she already has so much already though in my opinion ╮ （￣へ￣）╭
Good old jealousy. Here’s a list:
1. She’s jealous of the fact that their mother seem to care more about her sister than she does about her.
2. As the first born, her sister (our MC) is the one would inherit the dukedom.
3. The next best position from inheriting the dukedom would be to be part of the crown family (meaning marrying the crown prince), which even her sister normally would have dibs on (since she’s first born), unless the crown prince chooses otherwise.
i've forgot already but why did the sister want to kill the girl in the fist place. i don't want to read this all over again..