Isn't Seiji, who is the younger brother, the alpha, and Aiji the omega? At least that was what was stated in chapter 3.
That's what I thought, too.
Seiji is the omega and younger brother. Aiji is the alpha and older brother
bullshit
Nope, Seiji is the alpha and Aiji is the omega. Next chapters is going to show their childhood. You would know more.
Aiji is the one with the issues and being seen as different, not Seiji. Seiji is the kind one that likes Towa for understanding their differences.
http://www.mangago.zone/read-manga/ai_to_makoto_moriyo/uu/ai_to_makoto_moriyo-chapter-4/37/
http://www.mangago.zone/read-manga/ai_to_makoto_moriyo/uu/ai_to_makoto_moriyo-chapter-4/40/
Its clear that Seiji has issues about being a twin and being seen as different. I'm guessing that in the past, they were being compared. I'm also guessing that Seiji is the omega and Aiji is the alpha. He was probably ostracised in the past for being the omega and being seen as different from his twin brother, the alpha, makes him feel weak and pushed to the side.
This is my guess