In incest, there is the possibility of making an offspring with birth defects, but that can be avoided easily most of the time. Besides that, consent is a big issue. How do you know a father or mother didn't groom their child to want to have sex with them in the future? How do we know siblings weren't groomed to want to have sex with each other? Or if one of them groomed the other? Lastly, it can ruin the family dynamic or tear the family apart forever.
Such a silly argument lol. Very subjective. The same argument can be said about anal sex, if you want to poop everytime you sneeze then it's definitely unhealthy.
And why would anyone groom brother and sister to want to have sex? Your argument is crazy. The same argument can be said about two unrelated boys. What if they were groomed to be gay. Many gay men have been sexually abused as children which impacted on their sexuality. I know this personally because I've spoken to gay men.
You can't seperate the two I'm afraid.