I'm raising a daughter who is a bully!!

It's so embarrassing. Why???? I was a sweet little girl when I was a kid so I want to believe and I am wondering why is my daughter a BULLY!!!!!!. Okay, my daughter is only a bully to her brother. I feel sorry for the son because he's so sensitive from the beginning and looks up to his sister like an angel and then the girl claws comes out at him and he gets his feelings hurt big time and I have to hear about it with a screaming boy in my ear. Oh my, sigh........ I have tried everything like take her favorite toy away, timeouts, going to bed early and nothing has worked. NOTHING is working!!!! HELP!!!!! WHAT IS A MOTHER TO DO? I don't know what to do about her abusing her little brother. She's so mean and jealous of him. It's been going on for a few weeks and it's been getting ugly like I feel like putting my hands up in the air and walking away or hide in my bedroom but that's not going to help anything. She won't tell me why she abuses her brother. She likes waking him up early in the morning or going into his room during nap time but any other time she treats him like a diseased infested scum ball. My poor little boy and now he thinks hitting is funny which he gets in trouble for and then he cries because he's always getting his feelings hurt. I tried separating them during the day. It seemed like our happy home is a war zone and I don't know how to change the struggle to our fun happy space we once had. Oh I miss those days...okay we really never had a fun happy home and it's all in my head but I can dream can I? My ears hurt and it's all about me, right? I need a solution because I don't want my daughter to be mean and hurtful to her little brother anymore. Oh the tears........Take care, Peace!!!! I need some advice so if you have any please leave a comment and I'll happily will listen.....Please!!!!