Because the idea of wanting to fuck your brother, change your names and get married and make a family together is horrifying to most people?
Can you imagine how fucked up it would be to have to keep those feelings bottled up? To even go to the opposite end of the spectrum to allay suspicions? Bitterness coloring your feelings as you eventually blame your sibling for bringing out these impossible urges in you. At the same time you hate yourself and your situation because you're too much of a coward to be honest.
So you lash out, berating the object of your impossible affection. You want to hurt them like they've hurt you, but you just end up feeing even worse because you know it's not their fault.
They didn't make you love them.