For anyone still confused, Mamiya and Iori are not blood related!
Even if we assume Iori is indeed Seijuro's child, him and and Mamiya are actually not blood related because the grandfather (A) had Mamiya with another omega (B) and we find out later in the story that the grandmother (C) had an affair with another man (D) and gave birth to Seijuro!
For anyone still confused, Mamiya and Iori are not blood related!
Even if we assume Iori is indeed Seijuro's child, him and and Mamiya are actually not blood related because the grandfather (A) had Mamiya with another omega (B) and we find out later in the story that the grandmother (C) had an affair with another man (D) and gave birth to Seijuro!