Incestis legal in many parts of the world, often under certain conditions. Here is a map:
In some parts of the world it is fine for minors but not for adults. In others it is fine for consenting adults. In many countries, it is illegal.
Consensual incest between consenting adults in Japan is allowed. There are reports of it being relatively common. There are even tales of mother’s teaching their sons the practicalities of sex in a very direct way. I have no idea if these tales are true, but, as we know, everything we read on the web is totally true.
Anyway, Japan will, of course, turn anything into a game show, including, apparently, incest. There are all sorts of variations – fathers having to identify their daughters, for example, by examining breasts, bums and vaginas (inside and out), or sons with their mothers.
The prize seems to be a holiday to Hawaii. Lose, and genuine incest on screen is the penalty.
In the example above, a father has identified his daughter. Here’s another with a mother and son.
No idea if it’s true, but it could be.