I'm not meaning to be a bitch... but... grow up a little bit. My sister is eight as well and she calls me fat, jerk, ugly, mean, and says I hate you multiple times a day when I don't do what she wants. Just tell her it's not okay to say that type of thing. if she gets a mouth, shut it out. Keep reminding her that she cannot say anything like that to you. Even though you love her, and might get into trouble for it, on one occasion, say "I hate you too, brat" or something like that. Give her a taste of her own medicine.
Why is she saying she hates you? Like, have you done anything to make her say that?
When I was younger I used to fight with my older brother and I used to sit there and tell him: "I hate you! You're the worst brother in the whole world and I hate you!"
And then my mom told me he cried about it later because he didn't want to be a bad big brother. Granted he still is a bad big brother, I don't say it to his face anymore.
Just tell her it's not okay to say that type of thing. if she gets a mouth, shut it out. Keep reminding her that she cannot say anything like that to you. Even though you love her, and might get into trouble for it, on one occasion, say "I hate you too, brat" or something like that. Give her a taste of her own medicine.