How to tell my dad i hate my step mom and she has to go!?

so about a year ago i accidently butt dialed my dad and he heard me complaining about her and things were fixed for about a month and then she got worse! she was nice and caring towards me when they got married and ever since my sister was born 6 years ago she hates me! she treats me like cinderella! shes grossly fat and so cheap! for christmas she got me a vaccum! a vaccum! im 16! i dont want a vacuum! and when i told my dad he said it must have been a mix up. he doesnt believe me with anything i say anymore and yesterday they went and got a new puppy without me and didnt let me help pick a name because i dont live with them! and its not like i can my mom has custody of me! Idk what to do anymore and i have no one too talk to and nobody believes me