Okay, I still don't know how to organize my thoughts in here, so this might be very chaotic. I apologize in advance lmao, please bear with me.
Let's start with step-siblings.
My main issue is not the actual romantic attraction between step-siblings. True, we can't control who we fall for, I just think it's strange when someone is attracted to something that society expressly teaches us not to like. Anyway, as I said, this is not the main issue. Is it weird? Yes, but realistically, if marriage and/or sex happened, nothing major would result of it, since there is no blood relation (if the siblings are half related by blood, then I think it's a different situation, as we'll discuss later).
The thing that is bad about this kink is that it is accepted enough to be treated as a joke by a lot of people. Like, great, you aren't kink shamed, but at the same time it feels weird that this kink is so widespread. This actually results in massive amounts of porn, and we all know how porn is made. So many women are forced to be in this industry, and the siblings kink leads to yet another major kink which is a real-life issue: underage kinks. This one deserves a separate chapter
As for actual blood-related siblings (or cousins for that matter):
Along with what I mentioned above, blood related people liking each other brings some issues. Again, we are not trained to like our siblings, cousins, etc. so why do so many people do? I don't know the answer. Anyway, many studies have shown that the chances of passing a recessive disease greatly increase when siblings have children. Like, if you know that you are likely to give your child a life-long disease which will make their life so much harder, why would you do it? It seems gross and irresponsible to me.
Okay, this was a lot more concise than I expected for a rant.
Let me know what you think. Have you ever had a child with your sibling?