Yup, I think they are half brothers.
They have the same alpha mother
Father of Miyauchi becomes mother of Jirou
Jirous mother got a maid pregnant so they're half brothers. Jirou's mother is Miyauchi's father. Hope that makes sense lol.
Jirou's papa stuck his pickle in Jirou's mama's donut hole. They made Jirou.
But before that, Jirou's mama had already stuck her pickle in a servant's donut hole and made Miya.
Lmao..omg i choked on my coffee..(≧∀≦)
bless.
Okay, so at the beginning of chapter 10.2 they showed a family tree. Maybe I missed it, but... WHAT THE HELL FO YIU MEAN MIYAUCHI AND JIROU AND RELATED, JUST- WHAT!? ヽ(`Д´)ﾉ