I'm same as greatninja3, and as others commented it's probably really rare to actually be sexually attracted to your sibling. At least one you grew up with.
I don't really understand it. It's probably not a genetic thing, but also might not be a close proximity thing. Why? Well childhood friends end up falling for each other all the time but I suppose they aren't as frequently together as siblings in the early years.
I guess all I can say is the universe, life, the human mind? It's all complicated beyond imagination. People have fallen in love with their half siblings that they didn't know were siblings. I remember the general consensus that peeping on your female family members was a thing? I never have nor wanted to, and don't really understand if others did or that was just a common myth. I suppose before the internet it probably was common, just because access to naked females was a challenge.