Blackmail is a crime, but that doesn't really help you, You said your family has already seen the video and wants to disown you, right? This chick has nothing left to blackmail you with. You didn't say what you were doing on the video, but I'm sure you learned a lesson. Don't do things in public you wouldn't want others to see. Cameras are everywhere these days and privacy is almost a thing of the past. All you can do now is admit you were wrong and beg your family for forgiveness. If they disowned you, they are disappointed and suffering just as much as you are.
I think the best thing is to come clean with your family about what's going on and if she has any incriminating information, your family would prefer knowing it from you. Talk to a friend too. You need all the help you can get. This link will help you with ideas of how to handle things http://www.wikihow.com/Deal-With-Blackmail.
Wow. I don't know shat shed have on you to black mail you with. I've made plenty of stupid mistakes in my life but I've tried to live a reasonably honest life. There's not anything someone could black mail me with.
Perhaps your use of the term "chick" gives a clue to your general attitude. Hmmmm. Just a thought. I'm sure I must be wrong.