Help with a step mom?

okay, so when i was little my mom died. no biggie, i was 3, i dont remember her. when i was 4 my dad remarried my step mom. im 15 now. when my half sis was born 10 years ago, my step mom stopped caring about me. i know that sounds harsh, but its true. since then, ive "received" 2 half brothers. My step mom makes me call her mom, and she only talks to me when she needs me to do something or to yell at me when i do something wrong, or when one the my half siblings tell her i "hurt them all the time", which i dont, i avoid them cuz theyell say something like that. last febuary igot tired of the way she had been treating me so i started yelling at her saying how i hate her and hoe i hate living here and how im gonna runn away. she through me in my room and when i tyred to walk past her to leave the house, she slapped my across the face. she TOLD me " i dont like you but were stuck with each other so get used to it. " then she went crying to my dad saying how i was punching her and how i was being "ungodly" and had "the devil" in me. i was like WTF!!!! But my dads her b*tch and does watever she says. so now im grounded to August. thats 7 MONTHS! AND she toold the little ones that i was gonna hurt them and now there all scared of me and can get away with saying i hurt them even easier. They were gonna trow me out, but my yuoth paster was like, wtf, ur idoits, and they let me stay. and noe its mothers day and SHE got all upset cuz i didnt do sh*t for her. i really dont know waht to do.... am i doing something wrong? is she? whos at fault here? wat shud i do? plz, im desperate