The most horrible (effing) sister?

Okay, I have the worst sister ever. She likes to eat my food that I buy for myself, and she weighs more than me. She will throw stuff at me outside, and then when I come inside, she says I flipped her off. She will take my clothes that I hung up and throw them on the ground and then say my room isn't clean, and this was just today! She has done this all my life. She is my half sister. We live with our grandparents. We also have another brother is younger than us and she treats him even worse than me! My grandparents know she is a demon, but they don't know what to do with her. I'm 15, she's 11. She has a gluttony problem, and she is also a compulsive liar. What should I do?? Please help!