Start putting her on notice.........and tell your son to do the same.....if she doesn't help, she is grounded.......if she takes something and isn't given permission, she is grounded......take away her favorite whatever....cell phone, TV, going out, etc.......
Tell your son that he has to stand up to people like her, sister or not......he can't be taken advantage of like that.........
I think it's time to talk to them both together......let him tell her, with you there what upsets him that she does........
Also, you might take her to a place where she has to volunteer to help people less fortunate than you are........a Children's home, or homeless shelter where she can see how people need each other and how it is beneficial to help others......any place where it shows that families need to be able to count on each other and love each other.......
She's at that age where it is going to make a difference to her world........and right now, that's all she sees, is her world.....
Good Luck and hang in there.