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

Ah, yes. Incest