Fvking ur brother from another mother is always ok

So I thought about this and I have come to the conclusion to address the elephant in the room that's always been weighing on our minds "it's not ok to fvk ur bro" like srs? U why is fiddling with a friend u call bro such a bad thing? If it's consensual then it's fine, why do ppl act like it's such a big deal, when it's just like fvking with a close friend stripper that works at McDonald's overnight?