My daughter is a nasty bully in and out of the house; I’m clueless on how to stop her

I have two girls. My oldest is actually adopted from a family member, she was relinquished at birth and is 19 now. My youngest is 16, and my only with my ex-husband. She doesn’t see her father because he’s an awful person, he’s downright abusive and nasty. My youngest is turning into her father.She picks fights with my oldest when she’s bored and goes right for the jugular. The other day, I walked in on my youngest telling my oldest she was fat, ugly, and the reason her birth mother is addicted to drugs. I tried to shut it down, and my youngest just turned on me instead. I told her she was grounded, tried to make her go to her room, but she won’t listen. I take away her phone, her computer, she just finds them and takes them back. She holds her sister hostage to get what she wants. She’ll literally hold her by the hair and say she’ll only let go if I give back her phone. I tell her that this behavior isn’t okay, she needs to be respectful, and she’s behaving abusively- and she tells me she doesn’t care. She’s pleasant at all other times of the day, perfectly fine, and then she gets in a mood and just goes for it.I’ve been called in for conferences with her principal because my daughter is cyber bullying kids at her school. She threatens them, tells them she wishes they were dead. She finds all this really personal stuff (we live in a small town, it’s not hard) and targets them for it. We’ve been told she’s on her last strike, which I think is far too generous.I don’t know what to do. I know all the answers seem so simple, but taking her phone doesn’t work. Telling her not to do it doesn’t work. I can’t get her through the door of a therapist’s office. I can’t even get her dad to tell her not to do it because he tells her it’s being an “alpha.” I can’t even talk to her because she insults and makes fun of me instead of opening up. Please help. Advice is so necessary. via /r/Parenting http://ift.tt/2F2eicR