They didn't outright say it, but if you pay attention to when Miyauchi's story was being told, he is actually Jirou's half-brother.
Miyauchi is the son of an omega maid and Jirou's alpha mother.
Mii is the son of jirou mother (father /alpha) before she married to jirou father . ( Mii mother was the beta maid in jiro mother family house)... so basically the half brothers
Sorry, correction, its beta maid. Mii's mom is beta, not omega.
Ooo ok thank you both very much!
In what way is Mii related to Jirou? I don't think I got that part... Thanks!