Ciara
As tough as it has been for you that your parents are no longer together, hating your dad's GF will just make you feel bad.
I doubt she wants to be anything more than friends with you. You have a mum and she knows that but she may want you to have some input in your sister's lives. Wouldn't you like to see them? they are part of your dad after all?
Hating is such a negative thing to feel darling, try to see the positive in this. 2 homes, 2 lots of birthday presents, 2 lots of xmas presents. And who knows, maybe your dad's GF can be someone you really do like and can be your ally when he gets on at you to do your shcool work?
How old are you?