How it Miyauchi and jirou related?

Miyauchi's "father" was a woman who abandoned Miyauchi's mother (a woman too) to marry Tarou and give bith to Jirou. Basically they're brothers or half brothers. Jirou's mother is Miyauchi's father.

Thank you so much!

Miyauchi's father was in an affair with Jirou's mother. His mother was a servant of Jirou's family.