I think the legal definition of incest (in Japan at least) is penetration between two close relatives. Things like touching, kissing, or being next to each other while having sex are all perfectly legal. So any time you see a man having sex with his "daughter" or "mother" or"sister" or whatever, that is probably always fake. But when you have two sisters, or a mother and daughter, having sex with various men while the other is in the room, most of that is probably real.