Responses

Were they? It sounds like the omega's mom married the alpha's dad, and then they got divorced. That would have made them ex-stepbrothers, so they are no longer legally family. Not the same as adopted brothers, and hence the chapter 1 with its talk of not having clear bond names (what do you call a stepsibling that is no longer a step?). Taboo sounding, but not really - they technically are "strangers" on paper.
All these arguments and no one's gonna mention anything about the fact they're adopted brothers