No, there really are states where it is legal to marry a first cousin. Second cousin is legal anywhere, I think.
I only think I'd have a problem with two people like LnC in your story getting together if they lived in the same house since they were like 10. Anywhere from about 14 on, even if they are adopted, they are at the stage where they are making different kind of relationships than brother and sister.
I think, therefore, I get bananas.
When in doubt, think about time travel conundrums. You'll confuse yourself so you can forget what you were in doubt about.
What's the difference between ignorance, apathy, and ambivalence?
I don't know and I don't care one way or the other.
| 
