Maybe. You're also supposed to naturally want to make babies, but not everyone has that desire/physical ability.
It's quite plausible that you can fall in love/be attracted to a sibling that you were separated from your whole life but then met later on (without knowing they had the same bloodline).
Besides, it might just apply to immediate family- from what I've gleaned, marrying your cousin used to be all the rage.
I'll admit, like I said, I don't find it particularly nice to think about...but if we're going to assess gay rights based on the same thing- two consenting adults who aren't hurting anyone or themselves- then we have to accept them in the same way. Otherwise the "gay is gross" argument is also valid. And of course, that'd be ridiculous.