Ok. If you get in a fight with him, over her, (or threaten him), it is very very unlikely that he will stop - he will continue to hit her and even do so to "get revenge" on you. DO NOT confront him - what, you think you'll slap him around a bit or threaten him and he'll suddenly stop hitting her? Hell no, it's going to continue.
The only possible solution is to contact the authorities. She is underage, and hitting her is child abuse. He doesn't hit her younger sisters? Well, wait 4 years and see what happens. I bet they'll get hit too.
Her sisters might get taken away. That's the risk you have to take. More likely he will do some time, if you've got proof. Try to videotape him in the act (you hide outside and tape through a window). When you do this, she must not provoke him through any physical means - only be in an argument.
Let's review. You can
(a) Threaten him or attack him.
(b) Have her go to the authorities.
(c) Do nothing.
If you do (a), you might get in trouble, and it is very very unlikely that he will stop. You might think (a) is the most manly option, but it is still a bad one. You can't protect her every second of every day. You can't protect her sisters in the future. Think about it - this is a guy who fucking BEATS his DAUGHTER. Do you think he's going to say "oh - shit, I better not do this any more, it's wrong" after you confront him? Hell no.
If you do (b), what is likely is the family will get put through some counseling. This may seem like a pussy way out, but it has a far higher likelihood of working than (a) does. If it continues, he will be arrested. Be sure you have evidence for (b), like video footage and/or bruises. If you take video footage, be 100% sure she is not threatening him physically - that wouldn't absolve him of blame, but it somewhat lessens his moral culpability. It might be that if you go this route they will no longer pay for her college education. So? I worked my way through college and she can too. It's a harder life, true, but the good that getting this man some help or taking him AWAY, would do, is far greater. She owes it to herself and to her sisters. Lastly, make HER go to the authorities. You don't want to meddle in this and get your feet dirty. You support her in every way possible, but you must make HER do the report.
If you do (c), it will continue. He doesn't hit her sisters now. Wait 4 years, especially once she is gone and not able to be the target any more.
I have tried to take a calm, rational look at this problem. I hope this has helped.
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