[He can't help but smile a little when she starts laughing, but--]

Ugh, I don't have the energy right now to talk about sibling complexes. But did you know there's a thing called genetic sexual attraction that actually causes individuals who are related but didn't grow up together to sometimes experience intense attraction? It's believed to be related to other studies that indicate features--physical or mental--that are similar to one's own are considered more appealing.