I need Advice asap

*Anyone with a step mom could probably help me out on this one* I live with my Step mom, Real dad, and step brother. I know I'll never add up to the exceptations of my step mom, she simply asks to much. I know she'll always love my step brother more and well that honestly seems to not fase me anymore .. I have a older sister, she's 19 and when she was 13 my step mom was so bad on her my sister cut. I don't get to see my sister often since she's moved out, I hate it cause my sister is the one I can always talk to about everything and anything. My step mom is starting to get worse, she treats me very very unfairly from my step brother and I don't think she loves me. I wanna move in with my sister but with my sister being 19 and providing for herself, I'm not sure if my sister could afford it:( I can be siting at the counter and my step mom will be like "why are you siting there?" And give me this look, I'll get up and go to my room, I wanna stand up to her but I'm scared of her please help, how can I stand up to her !!!!!