Bro the brothers have no relation to that said third party. the only reason you say that because they’re having sex with that person all in the proximity of each other, mind you, I think (U) really need to Google and re-evaluate on whatever mindset u conjured cuz legal definition of it is literally having intercourse with blooded relations. And they’re just having a threesome and making a pack to have a consensual Fair relationship with who they care about. Well to the least how I’m perceiving this manga
Google literally says, "Yes, for it to be considered a polyamorous relationship, the person must be dating both brothers at the same time.If the relationships happened one after the other, it would simply be consecutive dating, not polyamory. Polyamory requires multiple ongoing, simultaneous romantic relationships." Just as an FYI, and who cares? Its the authors chose to write the story as they intended, you don't have to read it or like it, just the way it is.

Ah, yes. Incest