Dear Roger: Monkey Is A Bitch?

Bitch. Its a word I have had directed at me so many times I simply scoff and blow it off and quite often say," Yes, you figured out my middle name, so what?" But for the life of me, I cannot abide or understand what compels my son to call my 8 year old daughter by that name. She really doesn't deserve it. Shes not a bad kid at all. Yes, I quite often joke about her being,"Evil" or the ringleader of chaos, but quite honestly, she is a very loving, very sweet little girl who adores her big brother and cant quite understand why he gets so mad at her.
She thinks he hates her. She is on the fast track to believing all men are mean and will either yell at her to,"Go Away", ignore her, call her names like,"Bitch" or worse, destroy things she loves or take them away.
Its been a powder keg around my home again and I am sick and tired of it. My little girl does not deserve it, and I don't know what I am going to do about it. I regret soo many things with my elder son, I should have gotten him in counseling when everything first happened to us. I should have demanded he get help for all his anger, but instead I let him try to handle it himself, and unfortunately he has done a terrible job of it.
I have found myself reverting to some of my old behaviors, telling the younger kids to be quiet and not bother him, trying to stay out of his way and appease him because he is so much bigger than me, and when he is angry and I have to go to work I tell my daughter to stay out of his way and quietly message me via private message if hes,"Getting too mean." My stress has been through the roof and my migraines have been back in force, just like they were when I was living in the worst of it. At times he is relentless in his verbal abuse of me, calling me to task for every single shortfall he feels I have, every failing and mistake he feels I have made, he makes fun of my stutters or stress and then sarcastically belittles me when I get upset. It feels like sometimes my ex is standing there in front of me just trying to push me into a heart attack. Today he was so relentless my 10 year old son began screaming at him to stop.
He will be cruel and hateful and then a little while later he will attempt to be kind and helpful, (honeymoon period), to get what he wants, usually money or some treat or control of the tv.
He needs help. I am trying to get it for him, but I worry about my daughter and what she is going to think about men as she grows up, at times it feels like there is just no hope and that I have failed all the way around in protecting my children from not only monsters, but from themselves.
I've told her that the very next time he calls her,"Bitch" or any other horrible name, she is to post on her Facebook page, "I wonder if my brother calls his girlfriend,"Bitch" like he calls me, because if he does, that's not very nice." At least then I know our secret will be right in his pastors lap and maybe some man  somewhere, will have some advice?