I have a bad step mother?

my mum and dad have split and now my dad is seeing some one else it doesn't bother me that he is seeing some one that isn't my mum its that the person he is seeing treats me like dog. shes teases me she says that i am fat i have a small muffin top but i am not that fat and she is really fat she thinks she knows every thing because she is a nurse. she always says i am immature but im not i have never misbehaved i have never been grounded or gotten a time out or am like her kid who is a brat her kids are really skinny and sporty where i like to read books and stay at home and my dad changes around her we are really laid back but with her he pretends to care if i have a small amount of Pepsi before breakfast how do i tell him nicely that i don't like his girlfriend?