How do I get my sister to quit treating my mother and me like slaves?

She's the queen of everything, she rules the kingdom of our home, she decides what we eat, how we treat her. When we try to get her to listen to suggestions, she screams and cries and basically makes us too afraid of her to want to say anything. She is taking away my free will, and the free will of the other people in the house. What the H@#$ am I s'posed to do?