A lot of good things and bad things exists in our society. Its upto us to pick the good ones and avoid the wrong ones. Three things to mention here.
1. Yes Incest exists in the society. Its a wrong act. And they are sick people.
2. You said you saw incest video clip, how far are you sure those are real?
3. What is the purpose of asking this question?
I am particularly keen to know why you are asking. If you mean to say if this really exists you also want to try out, then I have to say there are childsex, rape or beastality is also there, do you think those are good acts? If not, then don't focus on sickening acts.