She doesn't really hate him but my five year old daughter was very vocal about the alleged lack of attention due to her newborn brother. It shouldn't shock you that our child is extremely eloquent (yes, she won't shut up) so the first 2 weeks the complaints, the sassiness, the defiance was constant. And if you add to that the fact that I was a bit hormonal, feeling especially overwhelmed first thing in the morning and again in the evening, my daughter's behavior was very hard to handle. I felt very guilty to "impose" a sibling on her and "destroy" her perfect only child life. (I may be quoting my older brother here.)
"You have more fun with baby Alex."
"You love him more"
"Everyone is coming to see him and not me"
"He is getting all the attention."
Our positive rebuttals were constant but, we know our daughter well, and that girl wanted to use the "new baby" card to get away with a lot. We were having none of that. We stuck to our guns in terms of discipline. Whatever was not allowed before the baby wasn't allowed now. We involved her in his care taking but also kept her routine as intact as possible.
At week six, Amelia seems very proud of being an older sister. She is very helpful and doesn't freak out when we are cooing and cuddling her brother. She has her moments but now that Alex stares at her in what she swears is adoration, all seems to be falling into place. Ay Mama!