from what thresh prince said, though, it's still considered romantic. so if his species has the same concepts and mores about siblings and incest that most sentients have, it would still be incestuous.

that said, from my view it's more like they're as close as brothers and monkey mom feels protective of justin, not like they're actual brothers, and even if there were legal adoptions and everything they're still not BIOLOGICALLY related, so my people wouldn't think much of it, anyway.