Their parents are both Alphas. In this universe Alpha x Omega pairs are super rare and generally looked down upon. So far Misaki and Hiromu are the only pair shown. Alphas usually get with Alphas.
So they both male alphas? So alphas can get pregnant too?
Yes, they're in a world where "anyone can get pregnant" (it's mentioned in the Omegaverse Guide that shows up in the beginning of some chapters). That's why the grandpa made the comment that he thought about Hiromu getting with Matsuo. Basically Alphas are the "elite" and many have an old-school/ superiority mindset where they feel that Alphas should only be with Alphas, to ensure that their kids also come out as Alphas.
I see. I probably missed that point that this universe anyone can get pregnant. Thank you ⁄(⁄ ⁄·⁄ω⁄·⁄ ⁄)⁄
No prob. The first chapter has mistranslations so that's probably also the reason it's not as easy to follow, so it's not your fault.
I just realized the twins and tomohiro(?) has alpha-omega parents.