@ValkNightRain - thanks for the response. For some reason, that chapter only looked vaguely familiar when I re-read it. I know I read it before, but I guess it just didn't really stick in my mind.
So yeah - they aren't really sisters - they just have a sort of big sister/little sister relationship. And as Takkun notes, they have similar eyes. And similar hairstyles. And I suppose it's significant (though it doesn't come across much in English) that they have the same accent?