My step-sister I having problems with my dad.?

someone should let her know that your dad isn't trying to take her father's place. her mom should make it clear that she will not be getting back with the father, and that it's not your step sister's fault. her mom needs to be firm with her, and punish her when she acts out like that or ruins things, because if she doesn't, then it will only get worse. once she realizes that your dad isn't trying to take her dad's place, hopefully she'll start acting better.