He is not a product of incest, the father had an affair with a woman, and he(the eldest son)is the mistresses baby aka a "bastard". However, the younger brothers mother was so upset that the eldest boy was being raised in the household she did something to him that make him mentally disabled.
I'm still confused, Hanae (blonde haired woman, Ranzou's mother) told Mitsue (Ikurou's mother) "it was me who seduced him". Isn't 'him' in this context Ikurou's father? So Ranzou's parents are Hanae and and Ikurou's father.
And in the next scene, Mitsue says to Ikurou's father, "You and hanae...you and your sister" So is not Ikurou's father's sister Hanae? Thus Ranzou being a product of incest?
Ok so...eldest brother is or isn't a product of incest???
Sorry I kept trying to skip the sxx scenes. But my god their family was messed up...