Romantic attraction isn't limited to non-blood relatives. (though I think there are studies which show animals generally preferring to mate with non-siblings so you can argue that there's at least no affinity for mating with those with close blood).

The whole idea that you can't fall in love with your brother is because any detrimental/recessive genetic material is more likely to be expressed as a phenotype in your resultant offspring.

The natural order in which things should actually happen would be:

1) fall in love (implusive, involuntary)
2) decide whether you should act on that (logical, voluntary)

Even if we swap the order around, #2 is simply done in advance to suppress #1 when it happens. The purpose of #2 is still dependant on the existence of #1.

That said and all, I like the sister. What I don't get is how Asuna's "soul" is maintained at the moment. SAO's data is supposedly deleted.. so what is left? The Nervgear's connected to somewhere, and assuming Fairy Land is Akihito's "SAO v2", they'll have to link the two together somehow (and thereby expose Akihito's whereabouts/trail).

I can see the sister following him into the dive this time.. and having a run-in with Sachi? Asuna won't be the only one they're trying to save I'd imagine (though she'd be the top priority no doubt).

Kirito could show Asuna's dad the picture and get him to post-phone the "wedding" until she's rescued.. but with that scumbag controlling the virtual servers things could get pretty dangerous for the both of them (Kirito, Asuna).