Why does my mom hate me an love my lil brother?

My mom hates me. Im 15 and my mom is always treating my lil 7 year old brother like hes a king. Every time he does something to me and i tell my mom she makes it my fault. The other day i was eating pizza and he took one of my slices and i told my mom and she said "well thats what happens when you take his pizza". But i never took his pizza in my life. She just makes every little thing about me. As soon as he starts crying she yells at me even though she doesnt know why hes crying. He gets in trouble with my step-dad and gets yelled at and runs to his room crying and she runs up behind him and babies him. But when he yells at me she just yells more. Im sick and tired of being treated like the court jester and he gets treated like the king. can anyone tell me how to get my mom to love me again? (NOTE: i have tried being a good kid and listening to her but it dont work. she just yells at me and tells me to stop being a kiss ***)