Paid for it
Seven times in the past three years I have offered people money to engage in incest for my filming and viewing pleasure. Six times they accepted. It usually cost $1500 but the last cost $5000 and it was a hard negotiation. The one rejection just gave a nervous laugh and politely refused.
These were not "trailer trash" or drunks, but people living in nice neighborhoods with no drug or alcohol problems. I picked them because they were attractive.
Three of them were mother/son, one was aunt/nephew, one was a mother with two sons and a daughter, one was a "nuclear family" of two parents and a son and two daughters. All but the first would have been illegal without incest. The larger families engaged in bisexual behavior without prompting. Follow-up visits showed that all but one of the mother/son pairs continued incest behavior in some form or other afterwards.
I think that most people have no real objection to incest, but they want to blame it on something. As long as there is an excuse or something where they can say "It's not my fault", they'll do it.
By next year I'll have saved up enough to find family number eight.