OP, I may have more sympathy for you than other posters, because I have also lived with a violent 16/17 yo DS, and had a younger sibling to protect as well as myself. I understand how the situation can worsen over months or even years in an odd, insidious, twisted way, so that you sort of slip into believing that appalling, abnormal, abusive, messed-up behaviour isn't really that bad. But it is. Really, your family situation is utterly awful.
As other posters have said, you talk as if your daughter is a problem, when in fact she has a problem. She is being badly abused in her own home, and her parents are failing to protect her. Of course she is disturbed.
You understand now that you must act, but I'm not sure that you are yet clear and confident about exactly what you must do. So I am offering you my benefit-of-hindsight advice here. You can get this situation back under control...
Here are the things you must do. Keep these in your head. They are all essential. If you fail at one, you will fail altogether.
- Stop the violence. Protect your daughter.
- Support your daughter to deal with the abuse she has been experiencing.
- Support your son to learn to control himself.
- Get support for yourself. This is hard and you need strength and help to make it happen.
1. Stop the violence. Protect your daughter.
Other people have said most of what needs saying. You must draw the line and tell your DC that there will be no more violence and you must call the police if there is. It took me months to reach this point, when my own DS was being violent towards me, so I understand how hard it is, and I understand that you don't want to do it, but (a) you have no other option and (b) it will work. I called 999 three times, and had my son arrested and charged on the 3rd. He has not been violent since. I know several other parents in this situation, and they have all found the same thing.
If by any chance your DS continues to be violent - and I really, really doubt it - then you will have to throw him out... Because although that will be awful for him (and you), it will be less awful than continuing to live with his violence.
2. Support your daughter to deal with the abuse she has been experiencing.
The first step is to act to stop the violence. You have absolutely no chance at all of supporting your DD in any other way if you do not do that, because until you do, for her you are part of the problem not part of the solution. Once she has seen you will protect her, after you have called 999, she may begin to accept other support from you, but it will take time - months or maybe years. Meanwhile you need to make sure she has support from someone else. Your first point of contact is your GP. Ask for a referral to counselling and CAMHS. Make absolutely certain she knows (and you may need help to understand this yourself) that you want to help her get support because she has had awful experiences, not because there is something 'wrong' with her. Family therapy may be useful too, but your DD needs support that is hers: at the moment you may have to accept that she has little reason to want to 'fix' the family and may prefer to leave it. cory's words - the apology and the promise - are the starting point for rebuilding your relationship with her. My younger son (who was not the main target of DS1's violence, though he was pushed, shouted at and stolen from) is still sometimes angry with me for failing to stop it earlier, a year after the last violence, and still wants a lock on his door to protect his things.
3. Support your son to learn to control himself
I understand how it feels impossible to control a boy-man who has grown bigger than you. It is impossible: that is why you need the police. He must learn to control himself, but until he does, you must call the police to control him, if - and every single time - he is violent. You should understand that calling the police to control him if he can't control himself is a responsible and loving thing to do. He may be frightened by his own loss of control - my DS1 was - and although he's likely to be angry in the short-term (my DS kept saying "I can't believe you called the police on your own son!"), he clearly needs someone to step in and may later be grateful. My son, a year on, sees me as having supported him through a period of him "being a shit".
Certainly, long term, we owe it to our sons to help them learn to have loving relationships, not abusive ones.
You need to talk to your son, probably almost constantly, about how he's behaving. The chances are his 'moral compass' has gone awry. You need to give him clear, consistent moral messages like "It's not ok to hurt people", "You need to control yourself now", "Stop, change what you're doing, that's not ok", and "It's great when you do the right thing!" Ask him questions, too, that make him think and take responsibility, like "What do you need to do now?", "How can you do the right thing here?"
Your DS would probably benefit from counselling but, like my son, he will probably refuse. It may help him (and you) to understand why his behaviour is abusive... But first the priority is to stop it.
If there are other factors you can see interfering with your son's ability to control himself - e.g. drugs including steroids, anger about something, being abused himself - then you may have other problems you need to deal with too.
4. Get support for yourself
You need more help, advice, strength, confidence and practical back-up than you have had. There isn't enough support for parents of teenagers, and it may be hard to find, but try to seek it out. Take it from anywhere and everywhere you can get it: your husband, family, friends, GP, schools, camhs, police, youthwork service, social services, anywhere. I know it is especially hard to ask for support when you feel that you have failed, but I can tell you from experience that it is a relief when you do, that it helps (usually), that you will find other people have had similar experiences, and that services will see it as a good thing that you recognise your limits and can ask for help.
Also look after yourself. You need to be strong to deal with all this. You also need to give your children - and especially your DD - a role model, and teach them that individuals deserve respect and care and joy in their lives.
Good luck.