V.P.
I'm sure you'll take offense to this question, but did you bust up their marriage? Did you date him before they were divorced? It doesn't surprise me that the 8 year old accepts you (and please learn the difference between except and accept, because as you state it, you are leaving the kids out, not welcoming them to your home, which I think you mean to say). She was only 3 when you hooked up with her dad. But the teenager remembers her dad before you came along.
Teens are moody to begin with. But to have to accept the fact that her father has moved on from her family is difficult. To her, you will always be an outsider. No one here can know for sure how your relationship with her has developed and what blame lies with her and what blame lies with you. (ETA: your statement that her mother "obviously" did brainwashing strikes me as hysterical and judgmental, which I'm sure comes across to the teen). But if I were in her shoes, I can promise you I would not accept my father's new wife easily, especially at that age. And the sooner you understand that fact, the better, because not only are you an intruder to her, you also clearly don't understand her. Even if you are not the reason her parents split, you are the proof that it happened.