I think it’s depends really. A lot of people are also against sibling relationships because they usually have a power dynamic or a grooming mechanic in them. Older siblings (especially significantly) or the sibling more loved/respected by the parents may have more authority and thus the other sibling might not be able to say no. Also most of these situations would have to start young where a younger sibling would not have the autonomy to know what they’re getting into this grooming.
This might not be the case for some situations but the problem is you never really know. Hell, if the victim is raised that way from really young they might not even realize it. That’s the reason I disapprove of them most of the time besides the obvious DNA issues. Hope that helps!
Why are people so against them being blood related brothers. The only negative point about incest is that babies are born with defects from incest. But they can't even make babies and they love each other. Everyone is different has different notions of love. And even if it were heterosexual incest, as long as they don't produce any offsprings and it's consensual, it's fine and no one should judge their relationship. And one more thing, we are all products of extensive incest that happened thousands of years ago. We won't be here without that.