I could say, "there is an evolutionary purpose to us being repulsed at the thought of reproducing with our gender. If we regularly had sex with our own gender, we would be...nothing. Our human race would disappear."
Not everyone is attracted to their siblings. Not everyone is attracted to their own gender. If a certain percentage of the population is attracted to their siblings, then it's okay. It has no effect on you. As long as brothers and sisters aren't reproducing children (because it would harm the child), then what's the big deal? Let them be. Accept those people.
It's not like pedophilia at all, where a minor's mind and body are being violated.
If you have two consenting siblings that want to have same-sex relations, who cares?
Just because you're not attracted to your siblings doesn't mean that everyone is repulsed at the thought, just like because 90% or whatever of the population is repulsed at the thought of homosexuality doesn't mean that everyone is repulsed at the thought.
It's amazing to me that you can have this line of thought as a gay man.