Oddly enough, I was literally *just* having a conversation about this kind of thing in a story I'm writing, in which a 16yo girl is very definitely not freaked out (as she probably would [should?] be in 'the real world') by the idea of having sex with her older brother (who, it should be noted, was likely her primary caregiver growing up), trying to figure out why she wouldn't be freaked, and you're onto something, I think, with the idea that Of course [the girls] are capable of consent--the very nature of the fantastic world in which they exist assures they are capable of anything.

*nods*

Otoh, I'd be interested in what you think of River, as someone of age, and obviously special, but so damaged as to make consent a serious issue for a long, long time.